Magnetic tape having characterized back coating layer, magnetic tape cartridge, and magnetic recording and reproducing apparatus

ABSTRACT

The magnetic tape includes: a non-magnetic support; a magnetic layer that includes ferromagnetic powder on one surface side of the non-magnetic support; and a back coating layer that includes non-magnetic powder on the other surface side of the non-magnetic support, in which the ferromagnetic powder is ferromagnetic powder selected from the group consisting of hexagonal strontium ferrite powder and ε-iron oxide powder, and the number of protrusions having a height of 50 nm or more and less than 75 nm on a surface of the back coating layer is 700 pieces/6400 μm2 or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2019-054337 filed on Mar. 22, 2019. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape, a magnetic tapecartridge, and a magnetic recording and reproducing apparatus.

2. Description of the Related Art

There are a tape-shaped magnetic recording medium and a disk-shapedmagnetic recording medium, and a tape-shaped magnetic recording medium,that is, a magnetic tape is mainly used for data storage applicationssuch as data backup and archive. As the magnetic tape, a magnetic tapethat includes a back coating layer on a surface side of a non-magneticsupport opposite to a surface side provided with a magnetic layer isknown (see JP2005-085305A).

SUMMARY OF THE INVENTION

JP2005-085305A discloses a magnetic tape containing ferromagnetic metalpowder in a magnetic layer as an example. With respect to this, inrecent years, hexagonal strontium ferrite powder and ε-iron oxide powderhave attracted attention as ferromagnetic powder to be used for magneticrecording from a viewpoint of high density recording suitability and thelike.

Therefore, the present inventors have studied a magnetic tape thatincludes a magnetic layer and a back coating layer, the magnetic layerincluding ferromagnetic powder selected from the group consisting ofhexagonal strontium ferrite powder and ε-iron oxide powder, and as aresult, a phenomenon in which dropout occurred remarkably was confirmed.Dropout is a signal reading failure, and occurrence of dropout increasesan error rate. Thus, it is required to reduce dropout.

An object of an aspect of the present invention is to provide a magnetictape that includes a magnetic layer and a back coating layer, themagnetic layer including ferromagnetic powder selected from the groupconsisting of hexagonal strontium ferrite powder and s-iron oxidepowder, and is capable of reducing dropout.

An aspect of the present invention relates to a magnetic tapecomprising: a non-magnetic support; a magnetic layer that includesferromagnetic powder on one surface side of the non-magnetic support;and a back coating layer that includes non-magnetic powder on the othersurface side of the non-magnetic support, in which the ferromagneticpowder is ferromagnetic powder selected from the group consisting ofhexagonal strontium ferrite powder and ε-iron oxide powder, and thenumber of protrusions having a height of 50 nm or more and less than 75nm on a surface of the back coating layer is 700 pieces/6400 μm² orless.

In one aspect, the number of protrusions having a height of 75 nm ormore on the surface of the back coating layer is 200 pieces/6400 μm² orless.

In one aspect, a difference (S_(after)−S_(before)) between a spacingSaner measured on a surface of the magnetic layer by opticalinterferometry after methyl ethyl ketone cleaning and a spacingS_(before) measured on the surface of the magnetic layer by opticalinterferometry before methyl ethyl ketone cleaning may be more than 0 nmand 15.0 nm or less. Hereinafter, the difference is also referred to asa “spacing difference before and after methyl ethyl ketone cleaning(S_(after)−S_(before))” or simply as a “difference(S_(after)−S_(before))”.

In one aspect, the magnetic tape may further comprise a non-magneticlayer including non-magnetic powder between the non-magnetic support andthe magnetic layer.

Another aspect of the present invention relates to a magnetic tapecartridge comprising: the magnetic tape described above.

Another aspect of the present invention relates to a magnetic recordingand reproducing apparatus comprising: the magnetic tape described above;and a magnetic head.

According to one aspect of the present invention, it is possible toprovide a magnetic tape that includes a magnetic layer and a backcoating layer, the magnetic layer including ferromagnetic powderselected from the group consisting of hexagonal strontium ferrite powderand ε-iron oxide powder, and is capable of reducing dropout. Accordingto one aspect of the present invention, it is possible to provide amagnetic recording and reproducing apparatus including such a magnetictape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Tape

An aspect of the present invention relates to a magnetic tapecomprising: a non-magnetic support; a magnetic layer that includesferromagnetic powder on one surface side of the non-magnetic support;and a back coating layer that includes non-magnetic powder on the othersurface side of the non-magnetic support, in which the ferromagneticpowder is ferromagnetic powder selected from the group consisting ofhexagonal strontium ferrite powder and ε-iron oxide powder, and thenumber of protrusions having a height of 50 nm or more and less than 75nm on a surface of the back coating layer is 700 pieces/6400 μm² orless.

Occurrence of dropout is affected by a recess on a magnetic layersurface, and this recess can be formed by a protrusion on a back coatinglayer surface biting into the magnetic layer surface (so-called“reflection”) in a state where the magnetic tape is wound.

The present inventors have intensively studied regarding the abovepoints, and further studied after obtaining a new knowledge that, in themagnetic tape that includes the magnetic layer including ferromagneticpowder selected from the group consisting of hexagonal strontium ferritepowder and ε-iron oxide powder, the protrusion on the back coating layersurface which can form a recess that greatly affects occurrence ofdropout on the magnetic layer is a protrusion having a height of 50 nmor more and less than 75 nm. As a result, it was newly found thatsetting the number of protrusions having a height of 50 nm or more andless than 75 nm on the back coating layer surface to 700 pieces/6400 μm²or less leads to reduction of dropout in the magnetic tape that includesthe magnetic layer including ferromagnetic powder selected from thegroup consisting of hexagonal strontium ferrite powder and ε-iron oxidepowder.

With respect to this, JP2005-085305A proposes that the number ofprotrusions having a height of 50 nm or more and less than 75 nm on theback coating layer surface is to be controlled in a range exceeding theabove range. However, the present inventors newly found that in a casewhere the number of protrusions having a height of 50 nm or more andless than 75 nm on the back coating layer surface is within such arange, it is difficult to suppress dropout in the magnetic tape thatincludes the magnetic layer including ferromagnetic powder selected fromthe group consisting of hexagonal strontium ferrite powder and ε-ironoxide powder. It is supposed that this is because the magnetic layerincluding ferromagnetic powder selected from the group consisting ofhexagonal strontium ferrite powder and ε-iron oxide powder generally hasa higher anisotropy magnetic field Hk than a magnetic layer includingferromagnetic powder used in a magnetic tape in the related art. Morespecific description is as follows. In a case where there is a recess onthe magnetic layer surface, a distance (spacing) between the magneticlayer surface and a magnetic head in a case where the magnetic layersurface and the magnetic head come into contact with each other to beslid on each other for reproducing data recorded on the magnetic layeris increased by a depth of the recess. Increase of the spacing can causea signal reading failure. Since the magnetic layer includingferromagnetic powder selected from the group consisting of hexagonalstrontium ferrite powder and ε-iron oxide powder generally has a highanisotropy magnetic field Hk, it is supposed that the magnetic layer iseasily affected even though increase of the spacing is slight.Therefore, it is considered that reduction of the number of protrusionshaving a height of 50 nm or more and less than 75 nm on the back coatinglayer surface from the range disclosed in JP2005-085305A, andsuppression of formation of a recess on the magnetic layer surface dueto the protrusions lead to reduction of dropout.

However, the above includes supposition of the present inventors, andthe present invention is not limited to this supposition. Furthermore,the present invention is not limited to other suppositions described inthis specification.

Hereinafter, the magnetic tape will be described more specifically. Inthe present invention and this specification, the “back coating layersurface (surface of the back coating layer)” of the magnetic tape hasthe same meaning as a surface of the magnetic tape on a back coatinglayer side, and the “magnetic layer surface (surface of the magneticlayer)” has the same meaning as a surface of the magnetic tape on amagnetic layer side.

Back Coating Layer

Number of Protrusions

The above magnetic tape includes a back coating layer includingnon-magnetic powder on a surface side of a non-magnetic support oppositeto a surface side provided with a magnetic layer. The number ofprotrusions having a height of 50 nm or more and less than 75 nm on thesurface of the back coating layer is 700 pieces/6400 μm² or less. From aviewpoint of further reducing dropout, the number of protrusions ispreferably 500 pieces/6400 μm² or less, more preferably 300 pieces/6400μm² or less, still more preferably 100 pieces/6400 μm² or less, stillmore preferably 50 pieces/6400 μm² or less, still more preferably 30pieces/6400 μm² or less, and still more preferably 10 pieces/6400 μm² orless. The number of protrusions can be, for example, 1 piece/6400 μm² ormore, 2 pieces/6400 μm² or more, or 3 pieces/6400 μm² or more. Since itis preferable that the number of protrusions is small from a viewpointof reducing dropout, the number of protrusions can be 0/6400 μm² ormore, or 0/6400 μm².

In addition, the number of protrusions having a height of 75 run or moreon the surface of the back coating layer is preferably 200 pieces/6400μm² or less, more preferably 100 pieces/6400 μm² or less, still morepreferably 50 pieces/6400 μm² or less, still more preferably 10pieces/6400 μm² or less, and still more preferably 5 pieces/6400 μm² orless. The number of protrusions having a height of 75 nm or more on thesurface of the back coating layer can be, for example, 0/6400 μm² ormore, 1 piece/6400 μm² or more, 2 pieces/6400 μm² or more, or 3pieces/6400 μm² or more.

The number of protrusions having a height of 50 nm or more and less than75 nm and the number of protrusions having a height of 75 nm or more onthe surface of the back coating layer are values measured in an area of80 μm×80 μm on the back coating layer surface by an atomic forcemicroscope (AFM). The number of protrusions is obtained as an arithmeticaverage of values in three different measurement regions on the backcoating layer surface. Examples of measurement condition include thefollowing measurement conditions. The number of protrusions shown in theexamples described later is obtained by measurement under the followingmeasurement conditions.

Non-Magnetic Powder of Back Coating Layer

The back coating layer is a layer including non-magnetic powder and abinding agent. The non-magnetic powder of the back coating layer can beone or both of carbon black and inorganic powder. For example, thenumber of protrusions on the back coating layer surface can becontrolled by using carbon black having an average particle size of 17to 50 nm (hereinafter, referred to as “fine particle carbon black”.) asthe non-magnetic powder of the back coating layer and adjusting thecontent thereof. Carbon black having an average particle size largerthan that of the above fine particle carbon black may be used incombination, or may not be used in combination. In a case of being usedin combination, for example, the number of protrusions on the backcoating layer can be controlled by using carbon black having an averageparticle size of 75 to 300 nm (hereinafter, referred to as “coarseparticle carbon black”.) in combination with the above fine particlecarbon black and adjusting a mixing ratio of both carbon blacks. In theback coating layer, regarding the content of the carbon black in theback coating layer, the total amount of the carbon black can be, forexample, in a range of 30 to 70 mass %, and more preferably in a rangeof 40 to 60 mass % with respect to the total solid content of the backcoating layer. In a case where the fine particle carbon black having anaverage particle size of 17 to 50 nm and the coarse particle carbonblack having an average particle size of 75 to 300 nm are used incombination, the mixing ratio thereof is preferably in a range of theformer:the latter=98:2 to 75:25, and more preferably in a range of 97:3to 85:15.

The back coating layer may or may not include one or more types ofinorganic powder, together with carbon black. As inorganic powder,inorganic powder having an average particle size of 5 to 250 nm and aMohs hardness of 5 to 9 can be used, for example. As inorganic powder,non-magnetic powder generally used for a non-magnetic layer,non-magnetic powder generally used as an abrasive for a magnetic layer,or the like can be used, among these, α-iron oxide, α-alumina, or thelike is preferable. The content of the inorganic powder in the backcoating layer is preferably in a range of 0 to 40.0 parts by mass, morepreferably in a range of 0 to 35.0 parts by mass, and still morepreferably in a range of 0 to 30.0 parts by mass with respect to 100.0parts by mass of the binding agent.

The back coating layer includes the above non-magnetic powder, caninclude a binding agent, and can optionally include one or moreadditives. In regards to the binding agent and the additive that canincluded in the back coating layer, the well-known technique regardingthe back coating layer can be applied, and the well-known techniqueregarding the treatment of the magnetic layer and/or the non-magneticlayer can be applied. For example, for the back coating layer,descriptions disclosed in paragraphs 0018 to 0020 of JP2006-331625A andpage 4, line 65 to page 5, line 38 of U.S. Pat. No. 7,029,774B can bereferred to. As a dispersing agent of the back coating layer, forexample, a fatty acid having 8 to 18 carbon atoms such as caprylic acid,lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid,elaidic acid, and linoleic acid, copper oleate, copper phthalocyanine,barium sulfate, a basic organic dye compound, and the like can be used.These may be used alone or in combination of two or more. The content ofthe dispersing agent of the back coating layer can be, for example, 0.5to 20.0 parts by mass with respect to 100.0 parts by mass of the bindingagent.

Magnetic Layer

Ferromagnetic Powder

The magnetic layer of the magnetic tape includes ferromagnetic powderselected from the group consisting of hexagonal strontium ferrite powderand ε-iron oxide powder. From a viewpoint of improving recordingdensity, it is preferable to use ferromagnetic powder having a smallaverage particle size. From this point, the average particle size offerromagnetic powder is preferably 50 nm or less, more preferably 45 nmor less, still more preferably 40 nm or less, still more preferably 35nm or less, still more preferably 30 nm or less, still more preferably25 nm or less, and still more preferably 20 nm or less. On the otherhand, from a viewpoint of magnetization stability, the average particlesize of ferromagnetic powder is preferably 5 nm or more, more preferably8 nm or more, still more preferably 10 nm or more, still more preferably15 nm or more, and still more preferably 20 nm or more.

In the magnetic layer of the magnetic tape, as ferromagnetic powder,only hexagonal strontium ferrite powder may be included, only ε-ironoxide powder may be included, or hexagonal strontium ferrite powder andε-iron oxide powder may be included. Hereinafter, the hexagonalstrontium ferrite powder and the ε-iron oxide powder will be furtherdescribed below.

Hexagonal Strontium Ferrite Powder

In the present invention and this specification, “hexagonal ferritepowder” refers to ferromagnetic powder in which a hexagonal ferrite typecrystal structure is detected as a main phase by X-ray diffractionanalysis. The main phase refers to a structure to which the highestintensity diffraction peak in an X-ray diffraction spectrum obtained byX-ray diffraction analysis is attributed. For example, in a case wherethe highest intensity diffraction peak is attributed to a hexagonalferrite type crystal structure in an X-ray diffraction spectrum obtainedby X-ray diffraction analysis, it is determined that the hexagonalferrite type crystal structure is detected as the main phase. In a casewhere only a single structure is detected by X-ray diffraction analysis,this detected structure is taken as the main phase. The hexagonalferrite type crystal structure includes at least an iron atom, adivalent metal atom and an oxygen atom, as a constituent atom. Thedivalent metal atom is a metal atom that can be a divalent cation as anion, and examples thereof may include an alkaline earth metal atom suchas a strontium atom, a barium atom, and a calcium atom, a lead atom, andthe like. In the present invention and this specification, hexagonalstrontium ferrite powder means that the main divalent metal atomincluded in the powder is a strontium atom. In addition, hexagonalbarium ferrite powder means that the main divalent metal atom includedin this powder is a barium atom. The main divalent metal atom refers toa divalent metal atom that accounts for the most on an at % basis amongdivalent metal atoms included in the powder. Here, a rare earth atom isnot included in the above divalent metal atom. The “rare earth atom” inthe present invention and this specification is selected from the groupconsisting of a scandium atom (Sc), an yttrium atom (Y), and alanthanoid atom. The Lanthanoid atom is selected from the groupconsisting of a lanthanum atom (La), a cerium atom (Ce), a praseodymiumatom (Pr), a neodymium atom(Nd), a promethium atom (Pm), a samarium atom(Sm), a europium atom (Eu), a gadolinium atom (Gd), a terbium atom (Tb),a dysprosium atom (Dy), a holmium atom (Ho), an erbium atom (Er), athulium atom (T m), an ytterbium atom (Yb), and a lutetium atom (Lu).

An activation volume of hexagonal strontium ferrite powder is preferablyin a range of 800 to 1500 nm³. The finely granulated hexagonal strontiumferrite powder having an activation volume in the above range issuitable for producing a magnetic tape exhibiting excellentelectromagnetic conversion characteristics. The activation volume of thehexagonal strontium ferrite powder is preferably 800 nm³ or more, forexample, 850 nm³ or more. Further, from a viewpoint of further improvingthe electromagnetic conversion characteristics, the activation volume ofthe hexagonal strontium ferrite powder is more preferably 1400 nm³ orless, still more preferably 1300 nm³ or less, still more preferably 1200nm³ or less, and still more preferably 1100 nm³ or less.

The “activation volume” is a unit of magnetization reversal and is anindex indicating the magnetic size of a particle. An activation volumedescribed in the present invention and this specification and ananisotropy constant Ku which will be described later are values obtainedfrom the following relational expression between a coercivity Hc and anactivation volume V, by performing measurement in an He measurementportion at a magnetic field sweep rate of 3 minutes and 30 minutes usinga vibrating sample magnetometer (measurement temperature: 23° C.±1° C.).For a unit of the anisotropy constant Ku, 1 erg/cc=1.0×10⁻¹ J/m³.Hc=2 Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2)}

[In the above formula, Ku: anisotropy constant (unit: J/m³), Ms:saturation magnetization (Unit: kA/m), k: Boltzmann constant, T:absolute temperature (unit: K), V: activation volume (unit: cm³), A:spin precession frequency (unit: s⁻¹), t: magnetic field reversal time(unit: s)]

An index for reducing thermal fluctuation, in other words, improvingthermal stability may include an anisotropy constant Ku. The hexagonalstrontium ferrite powder preferably has Ku of 1.8×10⁵ J/m³ or more, andmore preferably has Ku of 2.0×10⁵ J/m³ or more. Ku of the hexagonalstrontium ferrite powder may be, for example, 2.5×10⁵ J/m³ or less.Here, it means that the higher Ku is, the higher thermal stability is,this is preferable, and thus, a value thereof is not limited to thevalues exemplified above.

The hexagonal strontium ferrite powder may or may not include a rareearth atom. In a case where the hexagonal strontium ferrite powderincludes a rare earth atom, it is preferable to include a rare earthatom at a content (bulk content) of 0.5 to 5.0 at % with respect to 100at % of an iron atom. In an aspect, the hexagonal strontium ferritepowder including a rare earth atom may have a rare earth atom surfacelayer portion uneven distribution property. In the present invention andthis specification, the “rare earth atom surface layer portion unevendistribution property” means that a rare earth atom content with respectto 100 at % of an iron atom in a solution obtained by partiallydissolving hexagonal strontium ferrite powder with an acid (hereinafter,referred to as a “rare earth atom surface layer portion content” orsimply a “surface layer portion content” for a rare earth atom) and arare earth atom content with respect to 100 at % of an iron atom in asolution obtained by totally dissolving hexagonal strontium ferritepowder with an acid (hereinafter, referred to as a “rare earth atom bulkcontent” or simply a “bulk content” for a rare earth atom) satisfy aratio of a rare earth atom surface layer portion content/a rare earthatom bulk content>1.0. A rare earth atom content in hexagonal strontiumferrite powder which will be described later is the same meaning as therare earth atom bulk content. On the other hand, partial dissolutionusing an acid dissolves a surface layer portion of a particleconfiguring hexagonal strontium ferrite powder, and thus, a rare earthatom content in a solution obtained by partial dissolution is a rareearth atom content in a surface layer portion of a particle configuringhexagonal strontium ferrite powder. A rare earth atom surface layerportion content satisfying a ratio of “rare earth atom surface layerportion content/rare earth atom bulk content>1.0” means that in aparticle of hexagonal strontium ferrite powder, rare earth atoms areunevenly distributed in a surface layer portion (that is, more than aninside). The surface layer portion in the present invention and thisspecification means a partial region from a surface of a particleconfiguring hexagonal strontium ferrite powder toward an inside.

In a case where hexagonal strontium ferrite powder includes a rare earthatom, a rare earth atom content (bulk content) is preferably in a rangeof 0.5 to 5.0 at % with respect to 100 at % of an iron atom. It isconsidered that a bulk content in the above range of the included rareearth atom and uneven distribution of the rare earth atoms in a surfacelayer portion of a particle configuring hexagonal strontium ferritepowder contribute to suppression of a decrease in a reproducing outputin repeated reproduction. It is supposed that this is because hexagonalstrontium ferrite powder includes a rare earth atom with a bulk contentin the above range, and rare earth atoms are unevenly distributed in asurface layer portion of a particle configuring hexagonal strontiumferrite powder, and thus it is possible to increase an anisotropyconstant Ku. The higher a value of an anisotropy constant Ku is, themore a phenomenon called so-called thermal fluctuation can be suppressed(in other words, thermal stability can be improved). By suppressingoccurrence of thermal fluctuation, it is possible to suppress a decreasein reproducing output during repeated reproduction. It is supposed thatuneven distribution of rare earth atoms in a particulate surface layerportion of hexagonal strontium ferrite powder contributes tostabilization of spins of iron (Fe) sites in a crystal lattice of asurface layer portion, and thus, an anisotropy constant Ku may beincreased.

Moreover, it is supposed that the use of hexagonal strontium ferritepowder having a rare earth atom surface layer portion unevendistribution property as a ferromagnetic powder in the magnetic layeralso contributes to inhibition of a magnetic layer surface from beingscraped by being slid with respect to the magnetic head. That is, it issupposed that hexagonal strontium ferrite powder having rare earth atomsurface layer portion uneven distribution property can also contributeto an improvement of running durability of the magnetic tape. It issupposed that this may be because uneven distribution of rare earthatoms on a surface of a particle configuring hexagonal strontium ferritepowder contributes to an improvement of interaction between the particlesurface and an organic substance (for example, a binding agent and/or anadditive) included in the magnetic layer, and, as a result, a strengthof the magnetic layer is improved.

From a viewpoint of further suppressing a decrease in reproducing outputduring repeated reproduction and/or a viewpoint of further improving therunning durability, the rare earth atom content (bulk content) is morepreferably in a range of 0.5 to 4.5 at %, still more preferably in arange of 1.0 to 4.5 at %, and still more preferably in a range of 1.5 to4.5 at %.

The bulk content is a content obtained by totally dissolving hexagonalstrontium ferrite powder. In the present invention and thisspecification, unless otherwise noted, the content of an atom means abulk content obtained by totally dissolving hexagonal strontium ferritepowder. The hexagonal strontium ferrite powder including a rare earthatom may include only one kind of rare earth atom as the rare earthatom, or may include two or more kinds of rare earth atoms. The bulkcontent in the case of including two or more types of rare earth atomsis obtained for the total of two or more types of rare earth atoms. Thisalso applies to other components in the present invention and thisspecification. That is, unless otherwise noted, a certain component maybe used alone or in combination of two or more. A content amount orcontent in a case where two or more components are used refers to thatfor the total of two or more components.

In a case where the hexagonal strontium ferrite powder includes a rareearth atom, the included rare earth atom may be any one or more of rareearth atoms. As a rare earth atom that is preferable from a viewpoint offurther suppressing a decrease in reproducing output in repeatedreproduction, there are a neodymium atom, a samarium atom, a yttriumatom, and a dysprosium atom, here, the neodymium atom, the samariumatom, and the yttrium atom are more preferable, and a neodymium atom isstill more preferable.

In the hexagonal strontium ferrite powder having a rare earth atomsurface layer portion uneven distribution property, the rare earth atomsmay be unevenly distributed in the surface layer portion of a particleconfiguring the hexagonal strontium ferrite powder, and the degree ofuneven distribution is not limited. For example, for a hexagonalstrontium ferrite powder having a rare earth atom surface layer portionuneven distribution property, a ratio between a surface layer portioncontent of a rare earth atom obtained by partial dissolution underdissolution conditions which will be described later and a bulk contentof a rare earth atom obtained by total dissolution under dissolutionconditions which will be described later, that is, “surface layerportion content/bulk content” exceeds 1.0 and may be 1.5 or more. A“surface layer portion content/bulk content” larger than 1.0 means thatin a particle configuring the hexagonal strontium ferrite powder, rareearth atoms are unevenly distributed in the surface layer portion (thatis, more than in the inside). Further, a ratio between a surface layerportion content of a rare earth atom obtained by partial dissolutionunder dissolution conditions which will be described later and a bulkcontent of a rare earth atom obtained by total dissolution under thedissolution conditions which will be described later, that is, “surfacelayer portion content/bulk content” may be, for example, 10.0 or less,9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, 5.0 or less, or 4.0or less. Here, in the hexagonal strontium ferrite powder having a rareearth atom surface layer portion uneven distribution property, the rareearth atoms may be unevenly distributed in the surface layer portion ofa particle configuring the hexagonal strontium ferrite powder, and the“surface layer portion content/bulk content” is not limited to theillustrated upper limit or lower limit.

The partial dissolution and the total dissolution of the hexagonalstrontium ferrite powder will be described below. For the hexagonalstrontium ferrite powder that exists as a powder, the partially andtotally dissolved sample powder is taken from the same lot of powder. Onthe other hand, for the hexagonal strontium ferrite powder included inthe magnetic layer of the magnetic tape, a part of the hexagonalstrontium ferrite powder taken out from the magnetic layer is subjectedto partial dissolution, and the other part is subjected to totaldissolution. The hexagonal strontium ferrite powder can be taken outfrom the magnetic layer by a method described in a paragraph 0032 ofJP2015-091747A, for example.

The partial dissolution means that dissolution is performed such that,at the end of dissolution, the residue of the hexagonal strontiumferrite powder can be visually checked in the solution. For example, bypartial dissolution, it is possible to dissolve a region of 10 to 20mass % of the particle configuring the hexagonal strontium ferritepowder with the total particle being 100 mass %. On the other hand, thetotal dissolution means that dissolution is performed such that, at theend of dissolution, the residue of the hexagonal strontium ferritepowder cannot be visually checked in the solution.

The partial dissolution and measurement of the surface layer portioncontent are performed by the following method, for example. Here, thefollowing dissolution conditions such as an amount of sample powder areillustrative, and dissolution conditions for partial dissolution andtotal dissolution can be employed in any manner.

A container (for example, a beaker) containing 12 mg of sample powderand 10 mL of 1 mol/L hydrochloric acid is held on a hot plate at a settemperature of 70° C. for 1 hour. The obtained solution is filtered by amembrane filter of 0.1 μm. Elemental analysis of the filtrated solutionis performed by an inductively coupled plasma (ICP) analyzer. In thisway, the surface layer portion content of a rare earth atom with respectto 100 at % of an iron atom can be obtained. In a case where a pluralityof types of rare earth atoms are detected by elemental analysis, thetotal content of all rare earth atoms is defined as the surface layerportion content. This also applies to the measurement of the bulkcontent.

On the other hand, the total dissolution and measurement of the bulkcontent are performed by the following method, for example.

A container (for example, a beaker) containing 12 mg of sample powderand 10 mL of 4 mol/L hydrochloric acid is held on a hot plate at a settemperature of 80° C. for 3 hours. Thereafter, the method is carried outin the same manner as the partial dissolution and the measurement of thesurface layer portion content, and the bulk content with respect to 100at % of an iron atom can be obtained.

From a viewpoint of increasing the reproducing output in a case ofreproducing data recorded on the magnetic tape, it is desirable thatmass magnetization σs of the ferromagnetic powder included in themagnetic tape is high. In this regard, the hexagonal strontium ferritepowder including a rare earth atom but not having the rare earth atomsurface layer portion uneven distribution property tends to have σslargely lower than the hexagonal strontium ferrite powder including norare earth atom. On the other hand, it is considered that hexagonalstrontium ferrite powder having a rare earth atom surface layer portionuneven distribution property is preferable in suppressing such a largedecrease in σs. In an aspect, σs of the hexagonal strontium ferritepowder may be 45 A·m²/kg or more, and may be 47 A·m²/kg or more. On theother hand, from a viewpoint of noise reduction, σs is preferably 80A·m²/kg or less and more preferably 60 A·m²/kg or less. σs can bemeasured using a known measuring device, such as a vibrating samplemagnetometer, capable of measuring magnetic properties. In the presentinvention and this specification, unless otherwise noted, the massmagnetization σs is a value measured at a magnetic field intensity of 15kOe. 1 [kOe]=10⁶/4π[A/m]

Regarding the content (bulk content) of a constituent atom of thehexagonal strontium ferrite powder, a strontium atom content may be, forexample, in a range of 2.0 to 15.0 at % with respect to 100 at % of aniron atom. In an aspect, in the hexagonal strontium ferrite powder, adivalent metal atom included in the powder may be only a strontium atom.In another aspect, the hexagonal strontium ferrite powder may includeone or more other divalent metal atoms in addition to a strontium atom.For example, a barium atom and/or a calcium atom may be included. In acase where another divalent metal atom other than a strontium atom isincluded, a barium atom content and a calcium atom content in thehexagonal strontium ferrite powder are, for example, in a range of 0.05to 5.0 at % with respect to 100 at % of an iron atom, respectively.

As a crystal structure of hexagonal ferrite, a magnetoplumbite type(also called an “M type”), a W type, a Y type, and a Z type are known.The hexagonal strontium ferrite powder may have any crystal structure.The crystal structure can be checked by X-ray diffraction analysis. Inthe hexagonal strontium ferrite powder, a single crystal structure ortwo or more crystal structures may be detected by X-ray diffractionanalysis. For example, according to an aspect, in the hexagonalstrontium ferrite powder, only the M-type crystal structure may bedetected by X-ray diffraction analysis. For example, M-type hexagonalferrite is represented by a composition formula of AFe₁₂O₁₉. Here, Arepresents a divalent metal atom, and in a case where the hexagonalstrontium ferrite powder is the M-type, A is only a strontium atom (Sr),or in a case where, as A, a plurality of divalent metal atoms areincluded, as described above, a strontium atom (Sr) accounts for themost on an at % basis. The divalent metal atom content of the hexagonalstrontium ferrite powder is usually determined by the type of crystalstructure of the hexagonal ferrite and is not particularly limited. Thesame applies to the iron atom content and the oxygen atom content. Thehexagonal strontium ferrite powder may include at least an iron atom, astrontium atom, and an oxygen atom, and may further include a rare earthatom. Furthermore, the hexagonal strontium ferrite powder may or may notinclude atoms other than these atoms. As an example, the hexagonalstrontium ferrite powder may include an aluminum atom (Al). A content ofan aluminum atom can be, for example, 0.5 to 10.0 at % with respect to100 at % of an iron atom. From a viewpoint of further suppressing adecrease in reproducing output in repeated reproduction, the hexagonalstrontium ferrite powder includes an iron atom, a strontium atom, anoxygen atom, and a rare earth atom, and the content of atoms other thanthese atoms is preferably 10.0 at % or less, more preferably in a rangeof 0 to 5.0 at %, and may be 0 at % with respect to 100 at % of an ironatom. That is, in an aspect, the hexagonal strontium ferrite powder maynot include atoms other than an iron atom, a strontium atom, an oxygenatom, and a rare earth atom. The content expressed in at % is obtainedby converting a content of each atom (unit: mass %) obtained by totallydissolving hexagonal strontium ferrite powder into a value expressed inat % using an atomic weight of each atom. Further, in the presentinvention and this specification, “not include” for a certain atom meansthat a content measured by an ICP analyzer after total dissolution is 0mass %. A detection limit of the ICP analyzer is usually 0.01 parts permillion (ppm) or less on a mass basis. The “not included” is used as ameaning including that an atom is included in an amount less than thedetection limit of the ICP analyzer. In an aspect, the hexagonalstrontium ferrite powder may not include a bismuth atom (Bi).

In a case where the magnetic tape includes hexagonal strontium ferritepowder in the magnetic layer, the anisotropy magnetic field Hk of themagnetic layer is preferably 14 kOe or more, more preferably 16 kOe ormore, and still more preferably, 18 kOe or more. In addition, theanisotropy magnetic field Hk of the magnetic layer is preferably 90 kOeor less, more preferably 80 kOe or less, and still more preferably 70kOe or less.

The anisotropy magnetic field Hk in the present invention and thisspecification refers to a magnetic field in which magnetization issaturated in a case where a magnetic field is applied in a magnetizationhard axis direction. The anisotropy magnetic field Hk can be measuredusing a known measuring device, such as a vibrating sample magnetometer,capable of measuring magnetic properties. In the magnetic layerincluding hexagonal strontium ferrite powder, the magnetization hardaxis direction of the magnetic layer is an in-plane direction.

ε-Iron Oxide Powder

In the present invention and this specification, “ε-iron oxide powder”refers to ferromagnetic powder in which a ε-iron oxide type crystalstructure is detected as a main phase by X-ray diffraction analysis. Forexample, in a case where the highest intensity diffraction peak isattributed to a ε-iron oxide type crystal structure in an X-raydiffraction spectrum obtained by X-ray diffraction analysis, it isdetermined that the ε-iron oxide type crystal structure is detected asthe main phase. As a manufacturing method of the ε-iron oxide powder, amanufacturing method from a goethite, a reverse micelle method, and thelike are known. All of the manufacturing methods are well known.Regarding a method of manufacturing ε-iron oxide powder in which a partof Fe is substituted with substitutional atoms such as Ga, Co, Ti, Al,or Rh, a description disclosed in J. Jpn. Soc. Powder Metallurgy Vol. 61Supplement, No. S1, pp. 5280 to 5284, J. Mater. Chem. C, 2013, 1, pp.5200 to 5206 can be referred to, for example. Here, the manufacturingmethod of ε-iron oxide powder capable of being used as the ferromagneticpowder in the magnetic layer of the magnetic tape is not limited to themethods described here.

An activation volume of the ε-iron oxide powder is preferably in a rangeof 300 to 1500 nm³. The finely granulated ε-iron oxide powder having anactivation volume in the above range is suitable for producing amagnetic tape exhibiting excellent electromagnetic conversioncharacteristics. The activation volume of the ε-iron oxide powder ispreferably 300 nm³ or more, for example, 500 nm³ or more. Further, froma viewpoint of further improving the electromagnetic conversioncharacteristics, the activation volume of the ε-iron oxide powder ismore preferably 1400 nm³ or less, still more preferably 1300 nm³ orless, still more preferably 1200 nm³ or less, and still more preferably1100 nm³ or less.

An index for reducing thermal fluctuation, in other words, improvingthermal stability may include an anisotropy constant Ku. The ε-ironoxide powder preferably has Ku of 3.0×10⁴ J/m³ or more, and morepreferably has Ku of 8.0×10⁴ J/m³ or more. Ku of the ε-iron oxide powdermay be, for example, 3.0×10⁵ J/m³ or less. Here, it means that thehigher Ku is, the higher thermal stability is, this is preferable, andthus, a value thereof is not limited to the values exemplified above.

From a viewpoint of increasing the reproducing output in a case ofreproducing data recorded on the magnetic tape, it is desirable thatmass magnetization as of the ferromagnetic powder included in themagnetic tape is high. In this regard, in an aspect, σs of the ε-ironoxide powder may be 8 A·m²/kg or more, and may be 12 A·m²/kg or more. Onthe other hand, from a viewpoint of noise reduction, σs of the ε-ironoxide powder is preferably 40 A·m²/kg or less and more preferably 35A·m²/kg or less.

In a case where the magnetic tape includes ε-iron oxide powder in themagnetic layer, the anisotropy magnetic field Hk of the magnetic layeris preferably 18 kOe or more, more preferably 30 kOe or more, and stillmore preferably, 38 kOe or more. In addition, the anisotropy magneticfield Hk of the magnetic layer is preferably 100 kOe or less, morepreferably 90 kOe or less, and still more preferably 75 kOe or less. Inthe magnetic layer including ε-iron oxide powder, the magnetization hardaxis direction of the magnetic layer is an in-plane direction.

In the present invention and this specification, unless otherwise noted,an average particle size of various types of powder such asferromagnetic powder is a value measured by the following method using atransmission electron microscope.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, and the image is printed on printingpaper, is displayed on a display, or the like so that the totalmagnification ratio becomes 500,000 to obtain an image of particlesconfiguring the powder. A target particle is selected from the obtainedimage of particles, an outline of the particle is traced with adigitizer, and a size of the particle (primary particle) is measured.The primary particle is an independent particle which is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetic average of the particle sizes of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. An average particlesize shown in examples which will be described later is a value measuredby using a transmission electron microscope H-9000 manufactured byHitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted. In the present invention andthis specification, the powder means an aggregate of a plurality ofparticles. For example, ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. Further, the aggregate of theplurality of particles not only includes an aspect in which particlesconfiguring the aggregate directly come into contact with each other,but also includes an aspect in which a binding agent or an additivewhich will be described later is interposed between the particles. Theterm “particle” is used to describe powder in some cases.

As a method of taking sample powder from the magnetic tape in order tomeasure the particle size, a method disclosed in a paragraph of 0015 ofJP2011-048878A can be used, for example.

In the present invention and this specification, unless otherwise noted,(1) in a case where the shape of the particle observed in the particleimage described above is a needle shape, a fusiform shape, or a columnarshape (here, a height is greater than a maximum long diameter of abottom surface), the size (particle size) of the particles configuringthe powder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a plate shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetic average of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, and in a case of the definition (2), the average particle sizeis an average plate diameter. In a case of the definition (3), theaverage particle size is an average diameter (also referred to as anaverage particle diameter).

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably in a range of 50 to 90 mass % and morepreferably in a range of 60 to 90 mass %. Components other than theferromagnetic powder of the magnetic layer are at least a binding agentand may optionally include one or more additional additives. A highfilling percentage of the ferromagnetic powder in the magnetic layer ispreferable from a viewpoint of improving recording density.

Binding Agent and Curing Agent

The above magnetic tape may be a coating type magnetic tape, and mayinclude a binding agent in the magnetic layer. The binding agent is oneor more resins. As the binding agent, various resins usually used as abinding agent pf a coating type magnetic recording medium can be used.For example, as the binding agent, a resin selected from a polyurethaneresin, a polyester resin, a polyamide resin, a vinyl chloride resin, anacrylic resin obtained by copolymerizing styrene, acrylonitrile, ormethyl methacrylate, a cellulose resin such as nitrocellulose, an epoxyresin, a phenoxy resin, and a polyvinylalkylal resin such as polyvinylacetal or polyvinyl butyral can be used alone or a plurality of resinscan be mixed with each other to be used. Among these, a polyurethaneresin, an acrylic resin, a cellulose resin, and a vinyl chloride resinare preferable. The resin may be a homopolymer or a copolymer. Theseresins can be used as the binding agent even in the non-magnetic layerwhich will be described later and/or a back coating layer describedabove.

For the binding agent described above, descriptions disclosed inparagraphs 0028 to 0031 of JP2010-024113A can be referred to. An averagemolecular weight of the resin used as the binding agent can be, forexample, 10,000 or more and 200,000 or less as a weight-averagemolecular weight. The weight-average molecular weight of the presentinvention and this specification is a value obtained by performingpolystyrene conversion of a value measured by gel permeationchromatography (GPC) under the following measurement conditions. Theweight-average molecular weight shown in examples of a binding agentwhich will be described later is a value obtained by performingpolystyrene conversion of a value measured under the followingmeasurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mm inner diameter (ID)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In an aspect, a binding agent containing an active hydrogen-containinggroup can be used as the binding agent. The “active hydrogen-containinggroup” in the present invention and this specification refers to afunctional group that can form a cross-linked structure in a case wherethis group is subjected to curing reaction with a curable functionalgroup and a hydrogen atom contained in this group is eliminated.Examples of the active hydrogen-containing group include a hydroxygroup, an amino group (preferably a primary amino group or a secondaryamino group), a mercapto group, a carboxy group, and the like. Amongthese, a hydroxy group, an amino group, and a mercapto group arepreferable, and a hydroxy group is more preferable. In the binding agentcontaining the active hydrogen-containing group, an activehydrogen-containing group concentration is preferably in a range of 0.10meq/g to 2.00 meq/g. “eq” is an equivalent and is a unit that cannot beconverted into SI unit. Further, the active hydrogen-containing groupconcentration can be expressed by a unit “mgKOH/g”. In an aspect, in aresin containing the active hydrogen-containing group, an activehydrogen-containing group concentration is preferably in a range of 1 to20 mgKOH/g.

In an aspect, a binding agent containing an acidic group can be used asthe binding agent. The “acidic group” in the present invention and thisspecification is used in a meaning including a form of a group capableof releasing H⁺ in water or a solvent containing water (aqueous solvent)to be dissociated into an anion and a salt thereof. As a specificexample of an acidic group, a form of each of a sulfonic acid group(—SO₃H), a sulfuric acid group (—OSO3H), a carboxy group, a phosphoricacid group, and a salt thereof, can be used, for example. For example, aform of a salt of a sulfonic acid group (—SO₃H) means a grouprepresented by —SO₃M, where M represents a group representing an atom(for example, an alkali metal atom or the like) which can be a cation inwater or an aqueous solvent. The same applies to the form of each ofsalts of the various groups described above. As an example of a bindingagent containing an acidic group, a resin containing at least one typeof acidic group selected from the group consisting of a sulfonic acidgroup and a salt thereof (for example, a polyurethane resin, a vinylchloride resin, or the like) can be used, for example. Here, the resincontained in the magnetic layer is not limited to these resins. In thebinding agent containing an acidic group, an acidic group content maybe, for example, in a range of 0.03 to 0.50 meq/g. Contents of variousfunctional groups such as an acidic group included in a resin, can beobtained by a well-known method according to the kind of functionalgroup. The binding agent can be used in a magnetic layer formingcomposition in an amount of, for example, 1.0 to 30.0 parts by mass withrespect to 100.0 parts by mass of the ferromagnetic powder.

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in anaspect, a thermosetting compound which is a compound in which curingreaction (crosslinking reaction) proceeds due to heating can be used,and in another aspect, a photocurable compound in which a curingreaction (crosslinking reaction) proceeds due to light irradiation canbe used. Curing reaction of a curable functional group of the curingagent proceeds during a process of manufacturing a magnetic tape,whereby at least a part of the curing agent can be included in themagnetic layer in a state of being reacted (crosslinked) with othercomponents such as the binding agent. The same applies to the layerformed using this composition in a case where the composition used toform the other layer includes a curing agent. The preferred curing agentis a thermosetting compound, and polyisocyanate is suitable for this.For details of the polyisocyanate, descriptions disclosed in paragraphs0124 and 0125 of JP2011-216149A can be referred to. The curing agent canbe used in the magnetic layer forming composition in an amount of, forexample, 0 to 80.0 parts by mass, and preferably 50.0 to 80.0 parts bymass, from a viewpoint of improving a strength of the magnetic layer,with respect to 100.0 parts by mass of the binding agent.

Additive

The magnetic layer may include one or more kinds of additives, asnecessary. As the additives, the curing agent described above is used asan example. In addition, examples of the additive which can be includedin the magnetic layer include non-magnetic powder (for example,inorganic powder or carbon black), a lubricant, a dispersing agent, adispersing assistant, an antibacterial agent, an antistatic agent, andan antioxidant. For example, for the lubricant, descriptions disclosedin paragraphs 0030 to 0033, 0035, and 0036 of JP2016-126817A can bereferred to. The non-magnetic layer described later may include alubricant. For the lubricant which may be included in the non-magneticlayer, descriptions disclosed in paragraphs 0030, 0031, 0034, 0035, and0036 of JP2016-126817A can be referred to. For the dispersing agent,descriptions disclosed in paragraphs 0061 and 0071 of JP2012-133837A canbe referred to. For the additive of the magnetic layer, descriptionsdisclosed in paragraphs 0035 to 0077 of JP2016-051493A can be referredto. The dispersing agent may be added to a non-magnetic layer formingcomposition. For the dispersing agent which can be included in thenon-magnetic layer forming composition, a description disclosed in aparagraph 0061 of JP2012-133837A can be referred to. As the non-magneticpowder that can be included in the magnetic layer, non-magnetic powderwhich can function as an abrasive, or non-magnetic powder which canfunction as a protrusion forming agent which forms protrusions suitablyprotruded from the magnetic layer surface (for example, non-magneticcolloidal particles) is used. An average particle size of colloidalsilica (silica colloidal particle) shown in the examples described lateris a value obtained by a method disclosed in a paragraph 0015 ofJP2011-048878A as a method for measuring an average particle size. Asthe additive, a commercially available product can be suitably selectedor manufactured by a well-known method according to the desiredproperties, and any amount thereof can be used. Examples of the additivethat can be used to improve the dispersibility of the abrasive in themagnetic layer containing the abrasive include a dispersing agentdisclosed in paragraphs 0012 to 0022 of JP2013-131285A.

Spacing Difference (S_(after)−S_(before)) Before and After Methyl EthylKetone Cleaning

In an aspect, in the above magnetic tape, a difference(S_(after)−S_(before)) between a spacing S_(after) measured by opticalinterferometry after methyl ethyl ketone cleaning and a spacingS_(before) measured by optical interferometry before methyl ethyl ketonecleaning on a surface of the magnetic layer can be more than 0 nm and15.0 nm or less.

In the present invention and this specification, “methyl ethyl ketonecleaning” means that a sample piece cut out from a magnetic tape isimmersed in methyl ethyl ketone (200 g) at a liquid temperature of 20°C. to 25° C. and ultrasonically cleaned for 100 seconds (ultrasonicoutput: 40 kHz). A sample piece having a length of 5 cm is cut out froma magnetic tape to be cleaned and subjected to methyl ethyl ketonecleaning. A width of the magnetic tape and a width of the sample piececut from the magnetic tape are usually ½ inches (1 inch is 0.0254meters). For also magnetic tapes other than that having ½ inches width,a sample piece having a length of 5 cm may be cut out and subjected tomethyl ethyl ketone cleaning. A measurement of a spacing after methylethyl ketone cleaning which will be described in detail below isperformed after a sample piece after methyl ethyl ketone cleaning isleft under an environment of a temperature of 23° C. and a relativehumidity of 50% for 24 hours.

In the present invention and this specification, a spacing measured onthe magnetic layer surface of the magnetic tape by opticalinterferometry is a value measured by the following method.

In a state where the magnetic tape (specifically, the above samplepiece. The same applies hereinafter) and a transparent plate member (forexample, a glass plate or the like) are superposed such that themagnetic layer surface of the magnetic tape faces the transparent platemember, a pressing member is pressed at a pressure of 0.5 atm (1 atm is101325 Pa (Pascal)) from a side of the magnetic tape opposite to a sideof the magnetic layer. In this state, the magnetic layer surface of themagnetic tape is irradiated with light through the transparent platemember (irradiation region: 150,000 to 200,000 μm²), and a spacing(distance) between the magnetic layer surface of the magnetic tape and asurface of the transparent plate member on the magnetic tape side isobtained based on an intensity (for example, a contrast of aninterference fringe image) of interference light generated by an opticalpath difference between reflected light from the magnetic layer surfaceof the magnetic tape and reflected light from the surface of thetransparent plate member on the magnetic tape side. Here, emitted lightis not particularly limited. In a case where emitted light is lighthaving a light emission wavelength over a relatively wide wavelengthrange, such as white light having light with a plurality of wavelengths,a member, such as an interference filter, which has a function ofselectively cutting light with a specific wavelength or light out of aspecific wavelength region is disposed between the transparent platemember and a light receiving section that receives reflected light, andlight of some wavelengths or light in some wavelength regions inreflected light is selectively incident on the light receiving section.In a case where emitted light is light having a single emission peak(so-called monochromatic light), the member may not be used. As anexample, a wavelength of light incident on the light receiving sectioncan be in a range of 500 to 700 nm, for example. However, a wavelengthof light incident on the light receiving section is not limited to theabove range. Moreover, the transparent plate member may be a memberhaving transparency which allows emitted light to pass therethrough tosuch an extent that the interference light can be obtained byirradiating the magnetic tape with light through this member.

An interference fringe image obtained by the above spacing measurementis divided into 300,000 points to obtain a spacing of each point (adistance between the magnetic layer surface of the magnetic tape and thesurface of the transparent plate member on the magnetic tape side), andthus this is used as a histogram and a mode value in the histogram isused as a spacing. The difference (S_(after)−S_(before)) is a valueobtained by subtracting a mode value before methyl ethyl ketone cleaningfrom a mode value after methyl ethyl ketone cleaning at the above300,000 points.

Two sample pieces are cut out from the same magnetic tape, and thespacing value S_(before) is obtained without methyl ethyl ketonecleaning on one sample piece and the spacing value S_(after) is obtainedafter subjecting the other sample piece to methyl ethyl ketone cleaning.Thereby, the difference (S_(after)−S_(before)) may be obtained.Alternatively, the difference (S_(after)−S_(before)) may be obtained byobtaining the spacing value after subjecting the sample piece for whichthe spacing value is obtained before methyl ethyl ketone cleaning tomethyl ethyl ketone cleaning thereafter.

The above measurement can be performed using, for example, acommercially available tape spacing analyzer (tape spacing analyzer;TSA) such as tape spacing analyzer manufactured by Micro Physics.Spacing measurement in the examples was performed using a tape spacinganalyzer manufactured by Micro Physics.

In general, the magnetic layer surface includes a portion (protrusion)that mainly contacts (so-called true contact) the magnetic head in acase where the magnetic layer surface and the magnetic head come intocontact with each other to be slid on each other, and a portion(hereinafter, referred to as a “base portion”) that is provided lowerthan the portion. It is considered that the spacing is a value servingas an index of a distance between the magnetic head and the base portionin a case where the magnetic layer surface and the magnetic head slideon each other. Here, in a case where any component is present on themagnetic layer surface, it is considered that the spacing becomesnarrower as the amount of the component interposed between the baseportion and the magnetic head increases. On the other hand, in a casewhere the component is removed by methyl ethyl ketone cleaning, thespacing is widened, and thus the value of spacing S_(after) after methylethyl ketone cleaning becomes larger than the value of spacingS_(before) before methyl ethyl ketone cleaning. Therefore, it isconsidered that the spacing difference (S_(after)−S_(before)) before andafter methyl ethyl ketone cleaning can be used as an index of the amountof the component interposed between the base portion and the magnetichead.

With respect to the above point, the present inventors consider that thecomponent removed by methyl ethyl ketone cleaning may cause a change inspacing between the magnetic layer surface and the magnetic head in acase where the magnetic layer surface and the magnetic head come intocontact with each other to be slid on each other for reproducing datarecorded on the magnetic layer. It is supposed that in a case where sucha change in spacing can be suppressed, dropout can be further reduced.For this reason, it is considered that decrease of the spacingdifference (S_(after)−S_(before)) before and after methyl ethyl ketonecleaning, that is, reduction of the amount of the component contributesto further reduction of dropout by suppressing the change in spacing. Inthis regard, according to the study by the present inventors, there wasno correlation between a value of a spacing difference before and aftercleaning using an organic solvent other than methyl ethyl ketone, forexample, n-hexane, and the spacing difference (S_(after)−S_(before))before and after methyl ethyl ketone cleaning. It is supposed that thisis because the component cannot be removed or cannot be sufficientlyremoved with a solvent other than methyl ethyl ketone.

Details of the above component are not clear. As only supposition, thepresent inventors consider that the above component may be a componenthaving larger molecular weight than that of an organic compound normallyadded as an additive to the magnetic layer. The present inventorssuppose an aspect of this component as follows. In an aspect, themagnetic layer is formed by applying the magnetic layer formingcomposition containing the curing agent in addition to the ferromagneticpowder and the binding agent onto the non-magnetic support directly orvia another layer, and performing a curing treatment. By the curingtreatment here, the binding agent and the curing agent can be subjectedto curing reaction (crosslinking reaction). However, it is consideredthat a binding agent not subjected to curing reaction with the curingagent or a binding agent having insufficient curing reaction with thecuring agent is easily released from the magnetic layer and may also bepresent on the magnetic layer surface. The present inventors supposethat presence of such a binding agent on the magnetic layer surface mayalso cause a change in spacing between the magnetic layer surface andthe magnetic head in a case where the magnetic layer surface and themagnetic head come into contact with each other to be slid on eachother.

However, the above is supposition of the present inventors and does notlimit the present invention.

From a viewpoint of further reducing dropout, the difference(S_(after)−S_(before)) is preferably more than 0 nm and 15.0 nm or less.From a viewpoint of still further reducing dropout, the difference(S_(after)−S_(before)) is preferably 14.0 nm or less, more preferably13.0 nm or less, and still more preferably 12.0 nm or less. As will bedescribed in detail later, the difference (S_(after)−S_(before)) can becontrolled by a surface treatment of the magnetic layer in themanufacturing process of the magnetic tape. It is considered that in acase where the surface treatment of the magnetic layer is carried out sothat the difference (S_(after)−S_(before)) before and after methyl ethylketone cleaning is 0 nm, a large amount of the additive (for example, alubricant) of the magnetic layer is removed from the magnetic layer ofthe magnetic tape. Considering this point, the difference(S_(after)−S_(before)) is preferably more than 0 nm, more preferably 1.0nm or more, still more preferably 2.0 nm or more, still more preferably3.0 nm or more, and still more preferably 4.0 nm or more.

The magnetic layer described above can be provided directly on a surfaceof the non-magnetic support or indirectly through the non-magneticlayer.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The above magnetic tapemay have a magnetic layer directly on the surface of the non-magneticsupport, or may have a magnetic layer on the surface of the non-magneticsupport via a non-magnetic layer including non-magnetic powder.Non-magnetic powder used for the non-magnetic layer may be an inorganicpowder or an organic powder. In addition, carbon black and the like canbe used. Examples of the inorganic powder include powder such as metal,metal oxide, metal carbonate, metal sulfate, metal nitride, metalcarbide, and metal sulfide. The non-magnetic powder can be purchased asa commercially available product or can be manufactured by a well-knownmethod. For details thereof, descriptions disclosed in paragraphs 0146to 0150 of JP2011-216149A can be referred to. For carbon black which canbe used in the non-magnetic layer, descriptions disclosed in paragraphs0040 and 0041 of JP2010-024113A can be referred to. The content (fillingpercentage) of the non-magnetic powder of the non-magnetic layer ispreferably in a range of 50 to 90 mass % and more preferably in a rangeof 60 to 90 mass %.

The non-magnetic layer can include a binding agent, and can also includeone or more additives. In regards to other details of a binding agent oran additive of the non-magnetic layer, a well-known technology regardingthe non-magnetic layer can be applied. In addition, in regards to thetype and the content of the binding agent, and the type and the contentof the additive, for example, a well-known technology regarding themagnetic layer can be applied.

In the present invention and this specification, the non-magnetic layeralso includes a substantially non-magnetic layer including a smallamount of ferromagnetic powder as impurities, for example, orintentionally, together with the non-magnetic powder. Here, thesubstantially non-magnetic layer is a layer having a residual magneticflux density equal to or smaller than 10 mT, a layer having a coercivityequal to or smaller than 100 Oe, or a layer having a residual magneticflux density equal to or smaller than 10 mT and a coercivity equal to orsmaller than 100 Oe. It is preferable that the non-magnetic layer doesnot have a residual magnetic flux density and a coercivity.

Non-Magnetic Support

As the non-magnetic support (hereinafter, also simply referred to as a“support”), well-known components such as polyethylene terephthalate,polyethylene naphthalate, polyamide, polyamide imide, and aromaticpolyamide subjected to biaxial stretching are used. Among these,polyethylene terephthalate, polyethylene naphthalate, and polyamide arepreferable. A corona discharge, a plasma treatment, an easy-bondingtreatment, or a thermal treatment may be performed with respect to thesesupports in advance. For example, in a case where a protrusion is formedon the support surface by the non-magnetic powder contained in thesupport, the number of protrusions on the back coating layer surface canbe reduced by suppressing influence of the protrusion of thenon-magnetic support as the thickness of the back coating layerincreases. In addition, in a case where the support is manufactured by awell-known method, presence state of the protrusion on the supportsurface can be adjusted according to the size and the content of thenon-magnetic powder contained in the support.

Various Thicknesses

A thickness of the non-magnetic support is, for example, 3.0 to 80.0 μm,preferably in a range of 3.0 to 50.0 μm, and more preferably in a rangeof 3.0 to 10.0 μm.

A thickness of the magnetic layer can be optimized according to asaturation magnetization amount or a head gap length, and a band of arecording signal of the used magnetic head, and is, for example, 10 nmto 100 nm, and, from a viewpoint of high density recording, ispreferably in a range of 20 to 90 nm and more preferably in a range of30 to 70 nm. The magnetic layer may be at least a single layer, themagnetic layer may be separated into two or more layers having differentmagnetic properties, and a configuration of a well-known multilayeredmagnetic layer can be applied as the magnetic layer. A thickness of themagnetic layer in a case where the magnetic layer is separated into twoor more layers is a total thickness of the layers.

A thickness of the non-magnetic layer is, for example, 50 nm or more,preferably 70 nm or more, and more preferably 100 nm or more. On theother hand, the thickness of the non-magnetic layer is preferably 800 nmor less, and more preferably 500 nm or less.

A thickness of the back coating layer is preferably 0.9 μm or less, andmore preferably in a range of 0.1 to 0.7 μm.

Thicknesses of each layer of the magnetic tape and the non-magneticsupport can be obtained by a well-known film thickness measurementmethod. As an example, a cross section of the magnetic tape in athickness direction is exposed by known means such as an ion beam or amicrotome, and then a cross section observation is performed using ascanning electron microscope in the exposed cross section, for example.In the cross section observation, various thicknesses can be obtained asa thickness obtained at one portion of the cross section, or anarithmetic average of thicknesses obtained at a plurality of portions oftwo or more portions, for example, two portions which are randomlyextracted. In addition, the thickness of each layer may be obtained as adesigned thickness calculated according to manufacturing conditions.

Manufacturing Process

A composition for forming the magnetic layer, the non-magnetic layer,and the back coating layer usually contains a solvent together with thevarious components described above. As a solvent, various organicsolvents generally used for manufacturing a coating type magneticrecording medium can be used. The amount of the solvent in each layerforming composition is not particularly limited, and can be the same asthat of each layer forming composition of a normal coating type magneticrecording medium. A process of preparing each layer forming compositioncan generally include at least a kneading process, a dispersing process,and a mixing process provided before and after these processes asnecessary. Each process may be divided into two or more stages. Variouscomponents used for the preparation of each layer forming compositionmay be added at an initial stage or in a middle stage of each process.In addition, each component may be separately added in two or moreprocesses.

In order to prepare each layer forming composition, a well-knowntechnique can be used. In the kneading process, preferably, a kneaderhaving a strong kneading force such as an open kneader, a continuouskneader, a pressure kneader, or an extruder is used. Details of thekneading treatment are described in JP1989-106338A (JP-H01-106338A) andJP1989-079274A (JP-H01-079274A). Moreover, in order to disperse eachlayer forming composition, one or more kinds of dispersed beads selectedfrom the group consisting of glass beads and other dispersed beads canbe used as a dispersion medium. As such dispersed beads, zirconia beads,titania beads, and steel beads which are dispersed beads having a highspecific gravity are suitable. These dispersed beads can be used byoptimizing the particle diameter (bead diameter) and filling rate. As adispersing device, a well-known dispersing device can be used. For thenumber of protrusions on the back coating layer surface, as the smallerthe size of the non-magnetic powder used for forming the back coatinglayer is, the lower the dispersibility of a back coating layer formingcomposition is, and the protrusion tends to be easily formed on the backcoating layer surface. Since the dispersibility of the back coatinglayer forming composition can also be changed by combination of thenon-magnetic powder and the binding agent to be mixed for preparation ofthe back coating layer forming composition, the number of protrusions onthe back coating layer surface can also be controlled by combination ofthe non-magnetic powder and the binding agent. Further, in regards tokneading of the back coating layer forming composition, the smaller theamount of the solvent is, the lower the dispersibility of the kneadedproduct is, and the number of protrusions on the back coating layersurface tends to increase. In addition, in a case where the back coatinglayer forming composition is directly dispersed without kneading, thenumber of protrusions on the back coating layer surface tends todecrease. In regards to dispersion conditions, generally, in a casewhere the dispersion time during preparation of the back coating layerforming composition is increased, the number of protrusions on the backcoating layer surface tends to decrease. Each layer forming compositionsuch as the back coating layer forming composition may be filtered by awell-known method, before being subjected to a coating process. Thefiltering can be performed by using a filter, for example. As the filterused in the filtering, a filter having a pore diameter of 0.01 to 3 μm(for example, filter made of glass fiber or filter made ofpolypropylene) can be used, for example.

The magnetic layer can be formed, for example, by directly coating themagnetic layer forming composition onto the non-magnetic support orperforming multilayer coating of the magnetic layer forming compositionwith the non-magnetic layer forming composition in order or at the sametime. The back coating layer can be formed by coating the back coatinglayer forming composition to a side of the non-magnetic support oppositeto a side provided with the magnetic layer (or to be provided with themagnetic layer). For details of coating for forming each layer, adescription disclosed in a paragraph 0051 of JP2010-024113A can bereferred to.

After the coating process, various treatments such as a dryingtreatment, an orientation treatment of the magnetic layer, and a surfacesmoothing treatment (calendering treatment) can be performed. Forvarious processes, for example, well-known techniques disclosed inparagraphs 0052 to 0057 of JP2010-024113A can be referred to. Forexample, a coating layer of the magnetic layer forming composition ispreferably subjected to an orientation treatment while the coating layeris in a wet (undried) state. For the orientation treatment, the variouswell-known technologies such as descriptions disclosed in a paragraph0067 of JP2010-231843A can be used. For example, a vertical orientationtreatment can be performed by a well-known method such as a method usinga polar opposing magnet. In an orientation zone, a drying speed of thecoating layer can be controlled depending on a temperature and a flowrate of dry air and/or a transportation speed of the magnetic tape inthe orientation zone. The coating layer may be preliminarily driedbefore the transportation to the orientation zone. For the calenderingtreatment, in a case where the calendering condition is strengthened,the protrusions on the back coating layer surface tend to decrease.Examples of the calendering condition include a calendering pressure, acalendering temperature (a surface temperature of a calendering roll), acalendering speed, a hardness of a calendering roll, and the like. Asvalues of the calendering pressure, the calendering temperature, and thehardness of a calendering roll are increased, the calendering treatmentis strengthened, and as a value of the calendering speed is decreased,the calendering treatment is strengthened.

It is preferable to perform a heat treatment on the coating layer formedby coating the magnetic layer forming composition at any stage after thecoating process of the magnetic layer forming composition. As anexample, this heat treatment can be performed before and/or after thecalendering treatment. The heat treatment can be performed, for example,by placing the support on which the coating layer of the magnetic layerforming composition is formed under a heated atmosphere. The heatedatmosphere can be an atmosphere having an atmosphere temperature of 65°C. to 90° C., and is preferably an atmosphere having an atmospheretemperature of 65° C. to 75° C. This atmosphere can be, for example, anair atmosphere. The heat treatment under the heated atmosphere can beperformed, for example, for 20 to 50 hours. In an aspect, this heattreatment can allow curing reaction of the curable functional group ofthe curing agent to proceed.

An aspect of the manufacturing method of the magnetic tape can include amanufacturing method including wiping the magnetic layer surface with awiping material infiltrated with methyl ethyl ketone, preferably afterthe heat treatment (hereinafter, also referred to as a “methyl ethylketone wiping treatment”). By the methyl ethyl ketone wiping treatment,the value of the difference (S_(after)−S_(before)) described above canbe reduced. It is considered that presence of a component that can beremoved by the methyl ethyl ketone wiping treatment on the magneticlayer surface may also cause a change in spacing between the magneticlayer surface and the magnetic head in a case where the magnetic layersurface and the magnetic head come into contact with each other to beslid on each other.

The methyl ethyl ketone wiping treatment can be performed using a wipingmaterial infiltrated with methyl ethyl ketone instead of the wipingmaterial used in a dry wiping treatment, in accordance with the drywiping treatment generally performed in the manufacturing process of themagnetic recording medium. For example, before or after slitting themagnetic tape into a width that fits in a magnetic tape cartridge, themagnetic tape is run between a feeding roller and a winding roller, andthe wiping material (for example, clothes (for example, non-wovenfabrics) or papers (for example, tissue papers)) infiltrated with methylethyl ketone is pressed against the magnetic layer surface of therunning magnetic tape. Thereby, the methyl ethyl ketone wiping treatmenton the magnetic layer surface can be performed. A running speed of themagnetic tape and a tension applied to the longitudinal direction of themagnetic layer surface (hereinafter, simply referred to as a “tension”)in the above running can be the same as the processing conditiongenerally used by the dry wiping treatment generally performed in themanufacturing process of the magnetic recording medium. For example, therunning speed of the magnetic tape in the methyl ethyl ketone wipingtreatment can be about 60 to 600 m/min, and the tension can be about0.196 to 3.920 N (Newton). The methyl ethyl ketone wiping treatment canbe performed at least once.

A polishing treatment and/or the dry wiping treatment (hereinafter,referred to as a “dry surface treatment”.) generally performed in themanufacturing process of the coating type magnetic recording medium canbe performed on the magnetic layer surface one or more times. Accordingto the dry surface treatment, for example, it is possible to removeforeign matters, such as chips generated by the slit, generated duringthe manufacturing process and adhering to the magnetic layer surface. Ina case where the methyl ethyl ketone wiping treatment is performed, thedry surface treatment can be performed before and/or after the methylethyl ketone wiping treatment. Further, the back coating layer can bepolished by pressing polishing means such as a polishing tape or adiamond wheel against the back coating layer surface. The number ofprotrusions on the back coating layer surface can also be controlled bythe type of the polishing means used, the pressing pressure for pressingthe polishing means against the back coating layer surface duringpolishing, and the like.

It is possible to form a servo pattern in the manufactured magnetic tapeby a known method in order to enable tracking control of the magnetichead in the magnetic recording and reproducing apparatus, control of arunning speed of the magnetic tape, and the like. The “formation ofservo pattern” can also be referred to as “recording of servo signal”.Hereinafter, the formation of the servo pattern will be described.

The servo pattern is usually formed along a longitudinal direction ofthe magnetic tape. Examples of control (servo control) types using aservo signal include a timing-based servo (TBS), an amplitude servo, anda frequency servo.

As shown in a european computer manufacturers association (ECMA)-319, amagnetic tape (generally called “LTO tape”) conforming to a lineartape-open (LTO) standard employs a timing-based servo type. In thistiming-based servo type, the servo pattern is formed by continuouslydisposing a plurality of pairs of non-parallel magnetic stripes (alsoreferred to as “servo stripes”) in a longitudinal direction of themagnetic tape. In the present invention and this specification, a“timing-based servo pattern” refers to a servo pattern that enables headtracking in a timing-based servo system servo system. As describedabove, the reason why the servo pattern is formed of a pair ofnon-parallel magnetic stripes is to indicate, to a servo signal readingelement passing over the servo pattern, a passing position thereof.Specifically, the pair of magnetic stripes is formed so that an intervalthereof continuously changes along a width direction of the magnetictape, and the servo signal reading element reads the interval to therebysense a relative position between the servo pattern and the servo signalreading element. Information on this relative position enables trackingon a data track. Therefore, a plurality of servo tracks are usually seton the servo pattern along a width direction of the magnetic tape.

A servo band is formed of servo signals continuous in a longitudinaldirection of the magnetic tape. A plurality of servo bands are usuallyprovided on the magnetic tape. For example, in an LTO tape, the numberis five. A region interposed between two adjacent servo bands isreferred to as a data band. The data band is formed of a plurality ofdata tracks, and each data track corresponds to each servo track.

Further, in an aspect, as shown in JP2004-318983A, informationindicating a servo band number (referred to as “servo bandidentification (ID)” or “unique data band identification method (UDIM)information”) is embedded in each servo band. This servo band ID isrecorded by shifting a specific one of the plurality of pairs of theservo stripes in the servo band so that positions thereof are relativelydisplaced in a longitudinal direction of the magnetic tape.Specifically, a way of shifting the specific one of the plurality ofpairs of servo stripes is changed for each servo band. Accordingly, therecorded servo band ID is unique for each servo band, and thus, theservo band can be uniquely specified only by reading one servo band witha servo signal reading element.

Incidentally, as a method for uniquely specifying the servo band, thereis a method using a staggered method as shown in ECMA-319. In thisstaggered method, a group of pairs of non-parallel magnetic stripes(servo stripes) disposed continuously in plural in a longitudinaldirection of the magnetic tape is recorded so as to be shifted in alongitudinal direction of the magnetic tape for each servo band. Sincethis combination of shifting methods between adjacent servo bands isunique throughout the magnetic tape, it is possible to uniquely specifya servo band in a case of reading a servo pattern with two servo signalreading element elements.

As shown in ECMA-319, information indicating a position of the magnetictape in the longitudinal direction (also referred to as “longitudinalposition (LPOS) information”) is usually embedded in each servo band.This LPOS information is also recorded by shifting the positions of thepair of servo stripes in the longitudinal direction of the magnetictape, as the UDIM information. Here, unlike the UDIM information, inthis LPOS information, the same signal is recorded in each servo band.

It is also possible to embed, in the servo band, the other informationdifferent from the above UDIM information and LPOS information. In thiscase, the embedded information may be different for each servo band asthe UDIM information or may be common to all servo bands as the LPOSinformation.

As a method of embedding information in the servo band, it is possibleto employ a method other than the above. For example, a predeterminedcode may be recorded by thinning out a predetermined pair from the groupof pairs of servo stripes.

A head for forming a servo pattern is called a servo write head. Theservo write head has a pair of gaps corresponding to the pair ofmagnetic stripes as many as the number of servo bands. Usually, a coreand a coil are connected to each pair of gaps, and by supplying acurrent pulse to the coil, a magnetic field generated in the core cancause generation of a leakage magnetic field in the pair of gaps. In acase of forming the servo pattern, by inputting a current pulse whilerunning the magnetic tape on the servo write head, the magnetic patterncorresponding to the pair of gaps is transferred to the magnetic tape toform the servo pattern. A width of each gap can be appropriately setaccording to a density of the servo pattern to be formed. The width ofeach gap can be set to, for example, 1 μm or less, 1 to 10 μm, 10 μm ormore, and the like.

Before the servo pattern is formed on the magnetic tape, the magnetictape is usually subjected to a demagnetization (erasing) process. Thiserasing process can be performed by applying a uniform magnetic field tothe magnetic tape using a direct current magnet or an alternatingcurrent magnet. The erasing process includes direct current (DC) erasingand alternating current (AC) erasing. AC erasing is performed bygradually decreasing an intensity of the magnetic field while reversinga direction of the magnetic field applied to the magnetic tape. On theother hand, DC erasing is performed by applying a unidirectionalmagnetic field to the magnetic tape. As the DC erasing, there are twomethods. A first method is horizontal DC erasing of applying a magneticfield in one direction along a longitudinal direction of the magnetictape. A second method is vertical DC erasing of applying a magneticfield in one direction along a thickness direction of the magnetic tape.The erasing process may be performed on the entire magnetic tape or maybe performed for each servo band of the magnetic tape.

A direction of the magnetic field of the servo pattern to be formed isdetermined according to a direction of the erasing. For example, in acase where the horizontal DC erasing is performed to the magnetic tape,the servo pattern is formed so that the direction of the magnetic fieldis opposite to the direction of the erasing. Therefore, an output of aservo signal obtained by reading the servo pattern can be increased. Asshown in JP2012-053940A, in a case where a magnetic pattern istransferred to, using the gap, a magnetic tape that has been subjectedto vertical DC erasing, a servo signal obtained by reading the formedservo pattern has a monopolar pulse shape. On the other hand, in a casewhere a magnetic pattern is transferred to, using the gap, a magnetictape that has been subjected to horizontal DC erasing, a servo signalobtained by reading the formed servo pattern has a bipolar pulse shape.

The magnetic tape is usually accommodated in a magnetic tape cartridgeand the magnetic tape cartridge is mounted in the magnetic recording andreproducing apparatus.

Magnetic Tape Cartridge

Another aspect of the present invention relates to a magnetic tapecartridge comprising: the magnetic tape described above.

The details of the magnetic tape included in the above magnetic tapecartridge are as described above.

In the magnetic tape cartridge, generally, the magnetic tape isaccommodated inside a cartridge body in a state of being wound around areel. The reel is rotatably provided inside the cartridge body. As themagnetic tape cartridge, a single reel type magnetic tape cartridgehaving one reel inside the cartridge body and a dual reel type magnetictape cartridge having two reels inside the cartridge body are widelyused. In a case where the single reel type magnetic tape cartridge ismounted on a magnetic recording and reproducing apparatus for recordingand/or reproducing data on the magnetic tape, the magnetic tape ispulled out of the magnetic tape cartridge to be wound around the reel onthe magnetic recording and reproducing apparatus side. A magnetic headis disposed on a magnetic tape transportation path from the magnetictape cartridge to a winding reel. Feeding and winding of the magnetictape are performed between a reel (supply reel) on the magnetic tapecartridge side and a reel (winding reel) on the magnetic recording andreproducing apparatus side. During this time, data is recorded and/orreproduced as the magnetic head and the magnetic layer surface of themagnetic tape come into contact with each other to be slid on eachother. With respect to this, in the dual reel type magnetic tapecartridge, both reels of the supply reel and the winding reel areprovided in the magnetic tape cartridge. The above magnetic tapecartridge may be either a single reel type or a dual reel type magnetictape cartridge. The above magnetic tape cartridge has only to includethe magnetic tape according to one aspect of the present invention, andthe well-known technology can be applied to the others.

Magnetic Recording and Reproducing Apparatus

Another aspect of the present invention relates to a magnetic recordingand reproducing apparatus comprising: the magnetic tape described above;and a magnetic head.

In the present invention and this specification, the “magnetic recordingand reproducing apparatus” means an apparatus capable of performing atleast one of the recording of data on the magnetic tape or thereproducing of data recorded on the magnetic tape. Such an apparatus isgenerally called a drive. The magnetic recording and reproducingapparatus can be a sliding type magnetic recording and reproducingapparatus. The sliding type magnetic recording and reproducing apparatusis an apparatus in which the magnetic layer surface and the magnetichead come into contact with each other to be slid on each other, in acase of performing the recording of data on the magnetic tape and/orreproducing of the recorded data.

The magnetic head included in the magnetic recording and reproducingapparatus can be a recording head capable of performing the recording ofdata on the magnetic tape, or can be a reproducing head capable ofperforming the reproducing of data recorded on the magnetic tape. Inaddition, in an aspect, the magnetic recording and reproducing apparatuscan include both of a recording head and a reproducing head as separatemagnetic heads. In another aspect, the magnetic head included in themagnetic recording and reproducing apparatus can have a configurationthat both of an element for recording data (recording element) and anelement for reproducing data (reproducing element) are included in onemagnetic head. Hereinafter, the element for recording and the elementfor reproducing data are collectively referred to as an “element fordata”. As the reproducing head, a magnetic head (MR head) including amagnetoresistive (MR) element capable of sensitively reading datarecorded on the magnetic tape as a reproducing element is preferable. Asthe MR head, various known MR heads such as an anisotropicmagnetoresistive (AMR) head, a giant magnetoresistive (GMR) head, and atunnel magnetoresistive (TMR) head can be used. In addition, themagnetic head which performs the recording of data and/or thereproducing of data may include a servo signal reading element.Alternatively, as a head other than the magnetic head which performs therecording of data and/or the reproducing of data, a magnetic head (servohead) comprising a servo signal reading element may be included in themagnetic recording and reproducing apparatus. For example, a magnetichead that records data and/or reproduces recorded data (hereinafter alsoreferred to as “recording and reproducing head”) can include two servosignal reading elements, and the two servo signal reading elements canread two adjacent servo bands simultaneously. One or a plurality ofelements for data can be disposed between the two servo signal readingelements.

In the magnetic recording and reproducing apparatus, recording of dataon the magnetic tape and/or reproducing of data recorded on the magnetictape can be performed as the magnetic layer surface of the magnetic tapeand the magnetic head come into contact with each other to be slid oneach other. The magnetic recording and reproducing apparatus has only toinclude the magnetic tape according to one aspect of the presentinvention, and the well-known technology can be applied to the others.

For example, in a case of recording data and/or reproducing the recordeddata, first, tracking using a servo signal is performed. That is, bycausing the servo signal reading element to follow a predetermined servotrack, the element for data is controlled to pass on the target datatrack. Displacement of the data track is performed by changing a servotrack to be read by the servo signal reading element in a tape widthdirection.

The recording and reproducing head can also perform recording and/orreproducing with respect to other data bands. In this case, the servosignal reading element may be displaced to a predetermined servo bandusing the above described UDIM information, and tracking for the servoband may be started.

EXAMPLES

Hereinafter, the present invention will be described with reference toexamples. Here, the present invention is not limited to aspects shown inthe examples. “Parts” and “%” in the following description mean “partsby mass” and “mass %”, unless otherwise noted. The following processesand evaluation were performed in the air of 23° C.±1° C., unlessotherwise specified.

In Table 1 below, “SrFe1” and “SrFe2” represent hexagonal strontiumferrite powder, “ε-iron oxide” represents ε-iron oxide powder, and“BaFe” represents hexagonal barium ferrite powder having an averageparticle size of 21 nm.

An activation volume and an anisotropy constant Ku of various types offerromagnetic powder described below are values obtained by the methoddescribed above using a vibrating sample magnetometer (manufactured byToei Industry Co., Ltd.) for each ferromagnetic powder.

A mass magnetization σs is a value measured at a magnetic fieldintensity of 15 kOe using a vibrating sample magnetometer (manufacturedby Toei Industry Co., Ltd.).

Further, an anisotropy magnetic field Hk of the magnetic layer describedbelow is a value measured using a vibrating sample magnetometer of aTM-VSM5050-SMS type (manufactured by Tamagawa Co., Ltd.).

Method for Manufacturing Ferromagnetic Powder

Manufacturing Method 1 of Hexagonal Strontium Ferrite Powder

“SrFe1” shown in Table 1 is hexagonal strontium ferrite powdermanufactured by the following method.

1707 g of SrCO₃, 687 g of H₃BO₃, 1120 g of Fe₂O₃, 45 g of Al(OH)₃, 24 gof BaCO₃, 13 g of CaCO₃, and 235 g of Nd₂O₃ were weighed and mixed by amixer to obtain a raw material mixture.

The obtained raw material mixture was melted in a platinum crucible at amelting temperature of 1390° C., and a hot water outlet provided at abottom of the platinum crucible was heated while stirring a melt, andthe melt was discharged in a rod shape at about 6 g/sec. Hot water wasrolled and quenched by a water-cooled twin roller to manufacture anamorphous body.

280 g of the manufactured amorphous body was charged into an electricfurnace, was heated to 635° C. (crystallization temperature) at aheating rate of 3.5° C./min, and was kept at the same temperature for 5hours to precipitate (crystallize) hexagonal strontium ferriteparticles.

Next, a crystallized product obtained above including hexagonalstrontium ferrite particles was coarsely pulverized in a mortar, and1000 g of zirconia beads having a particle diameter of 1 mm and 800 mLof an acetic acid aqueous solution of 1% concentration were added to thecrystallized product in a glass bottle, to be dispersed by a paintshaker for 3 hours. Thereafter, the obtained dispersion liquid wasseparated from the beads, to be put in a stainless beaker. Thedispersion liquid was statically left at a liquid temperature of 100° C.for 3 hours and subjected to a dissolving process of a glass component,and then the crystallized product was sedimented by a centrifugalseparator to be washed by repeatedly performing decantation and wasdried in a heating furnace at an internal temperature of the furnace of110° C. for 6 hours to obtain hexagonal strontium ferrite powder.

The hexagonal strontium ferrite powder (“SrFe1” in Table 1) obtainedabove had an average particle size of 18 nm, an activation volume of 902nm³, an anisotropy constant Ku of 2.2×10⁵ J/m³, and a mass magnetizationσs of 49 A·m²/kg.

12 mg of sample powder was taken from the hexagonal strontium ferritepowder obtained above, elemental analysis of the filtrated solutionobtained by partially dissolving this sample powder under dissolutionconditions illustrated above was performed by an ICP analyzer, and asurface layer portion content of a neodymium atom was determined.

Separately, 12 mg of sample powder was taken from the hexagonalstrontium ferrite powder obtained above, elemental analysis of thefiltrated solution obtained by completely dissolving this sample powderunder dissolution conditions illustrated above was performed by an ICPanalyzer, and a bulk content of a neodymium atom was determined.

A content (bulk content) of a neodymium atom with respect to 100 at % ofan iron atom in the hexagonal strontium ferrite powder obtained abovewas 2.9 at %. A surface layer portion content of a neodymium atom was8.0 at %. It was confirmed that a ratio between a surface layer portioncontent and a bulk content, that is, “surface layer portion content/bulkcontent” was 2.8, and a neodymium atom was unevenly distributed in asurface layer of a particle.

The fact that the powder obtained above shows a crystal structure ofhexagonal ferrite was confirmed by performing scanning with CuKα raysunder conditions of a voltage of 45 kV and an intensity of 40 mA andmeasuring an X-ray diffraction pattern under the following conditions(X-ray diffraction analysis). The powder obtained above showed a crystalstructure of hexagonal ferrite of a magnetoplumbite type (M type). Acrystal phase detected by X-ray diffraction analysis was a single phaseof a magnetoplumbite type.

PANalytical X'Pert Pro analyzer, PIXcel detector

Soller slit of incident beam and diffracted beam: 0.017 radians

Fixed angle of dispersion slit: ¼ degrees

Mask: 10 mm

Anti-scattering slit: ¼ degrees

Measurement mode: continuous

Measurement time per stage: 3 seconds

Measurement speed: 0.017 degrees per second

Measurement step: 0.05 degrees

Manufacturing Method 2 of Hexagonal Strontium Ferrite Powder

“SrFe2” shown in Table 1 is hexagonal strontium ferrite powdermanufactured by the following method.

1725 g of SrCO₃, 666 g of H₃BO₃, 1332 g of Fe₂O₃, 52 g of Al(OH)₃, 34 gof CaCO₃, and 141 g of BaCO₃ were weighed and mixed by a mixer to obtaina raw material mixture.

The obtained raw material mixture was melted in a platinum crucible at amelting temperature of 1380° C., and a hot water outlet provided at abottom of the platinum crucible was heated while stirring a melt, andthe melt was discharged in a rod shape at about 6 g/sec. Hot water wasrolled and quenched by a water-cooled twin roller to manufacture anamorphous body.

280 g of the obtained amorphous body was charged into an electricfurnace, was heated to 645° C. (crystallization temperature), and waskept at the same temperature for 5 hours to precipitate (crystallize)hexagonal strontium ferrite particles.

Next, a crystallized product obtained above including hexagonalstrontium ferrite particles was coarsely pulverized in a mortar, and1000 g of zirconia beads having a particle diameter of 1 mm and 800 mLof an acetic acid aqueous solution of 1% concentration were added to thecrystallized product in a glass bottle, to be dispersed by a paintshaker for 3 hours. Thereafter, the obtained dispersion liquid wasseparated from the beads, to be put in a stainless beaker. Thedispersion liquid was statically left at a liquid temperature of 100° C.for 3 hours and subjected to a dissolving process of a glass component,and then the crystallized product was sedimented by a centrifugalseparator to be washed by repeatedly performing decantation and wasdried in a heating furnace at an internal temperature of the furnace of110° C. for 6 hours to obtain hexagonal strontium ferrite powder.

The obtained hexagonal strontium ferrite powder (“SrFe2” in Table 1) hadan average particle size of 19 nm, an activation volume of 1102 nm³, ananisotropy constant Ku of 2.0×10⁵ 0.1/m³, and a mass magnetization σs of50 A·m²/kg.

Method of manufacturing ε-Iron Oxide Powder

“ε-iron oxide” shown in Table 1 is ε-iron oxide powder manufactured bythe following method.

8.3 g of iron(III) nitrate nonahydrate, 1.3 g of gallium(III) nitrateoctahydrate, 190 mg of cobalt(II) nitrate hexahydrate, 150 mg oftitanium(IV) sulfate, and 1.5 g of polyvinylpyrrolidone (PVP) weredissolved in 90 g of pure water, and while the dissolved product wasstirred using a magnetic stirrer, 4.0 g of an aqueous ammonia solutionhaving a concentration of 25% was added to the dissolved product under acondition of an atmosphere temperature of 25° C. in an air atmosphere,and the dissolved product was stirred for 2 hours while maintaining atemperature condition of the atmosphere temperature of 25° C. A citricacid aqueous solution obtained by dissolving 1 g of citric acid in 9 gof pure water was added to the obtained solution, and the mixture wasstirred for 1 hour. The powder sedimented after stirring was collectedby centrifugal separation, was washed with pure water, and was dried ina heating furnace at a furnace temperature of 80° C.

800 g of pure water was added to the dried powder, and the powder wasdispersed again in water to obtain dispersion liquid. The obtaineddispersion liquid was heated to a liquid temperature of 50° C., and 40 gof an aqueous ammonia solution having a concentration of 25% wasdropwise added with stirring. After stirring for 1 hour whilemaintaining the temperature at 50° C., 14 mL of tetraethoxysilane (TEOS)was dropwise added and was stirred for 24 hours. Powder sedimented byadding 50 g of ammonium sulfate to the obtained reaction solution wascollected by centrifugal separation, was washed with pure water, and wasdried in a heating furnace at a furnace temperature of 80° C. for 24hours to obtain a ferromagnetic powder precursor.

The obtained ferromagnetic powder precursor was loaded into a heatingfurnace at a furnace temperature of 1000° C. in an air atmosphere andwas heat-treated for 4 hours.

The heat-treated ferromagnetic powder precursor was put into an aqueoussolution of 4 mol/L sodium hydroxide (NaOH), and the liquid temperaturewas maintained at 70° C. and was stirred for 24 hours, whereby a silicicacid compound as an impurity was removed from the heat-treatedferromagnetic powder precursor.

Thereafter, the ferromagnetic powder from which the silicic acidcompound was removed was collected by centrifugal separation, and waswashed with pure water to obtain a ferromagnetic powder.

The composition of the obtained ferromagnetic powder that was checked byhigh-frequency inductively coupled plasma-optical emission spectrometry(ICP-OES) has Ga, Co, and a Ti substitution type ε-iron oxide(ε-Ga_(0.58)Fe_(1.42)O₃). In addition, X-ray diffraction analysis isperformed under the same condition as that described above for themanufacturing method 1 of hexagonal strontium ferrite powder, and from apeak of an X-ray diffraction pattern, it is checked that the obtainedferromagnetic powder does not include α-phase and γ-phase crystalstructures, and has a single-phase and ε-phase crystal structure (ε-ironoxide type crystal structure).

The obtained ε-iron oxide powder (“ε-iron oxide” in Table 1) had anaverage particle size of 12 nm, an activation volume of 746 nm³, ananisotropy constant Ku of 1.2×10⁵ J/m³, and a mass magnetization σs of16 A·m²/kg.

Example 1

List of each layer forming composition is shown below.

List of Magnetic Layer Forming Composition

Magnetic Liquid

Ferromagnetic powder (type: see Table 1): 100.0 parts

Oleic acid: 2.0 parts

Vinyl chloride copolymer (MR 104 manufactured by Kaneka Corporation):10.0 parts

-   -   (weight-average molecular weight: 55,000, active        hydrogen-containing group (hydroxy group): 0.33 meq/g, OSO₃K        group (potassium salt of sulfuric acid group): 0.09 meq/g)

SO₃Na group-containing polyurethane resin: 4.0 parts

-   -   (weight-average molecular weight: 70,000, active        hydrogen-containing group (hydroxy group): 4 to 6 mgKOH/g, SO₃Na        group (sodium salt of sulfonic acid group): 0.07 meq/g)

Polyalkyleneimine polymer (synthetic product obtained by the methoddisclosed in paragraphs 0115 to 0123 of JP2016-051493A): 6.0 parts

Methyl ethyl ketone: 150.0 parts

Cyclohexanone: 150.0 parts

Abrasive liquid

α-alumina (Brunauer-emmett-teller (BET) specific surface area: 19 m²/g):6.0 parts

SO₃Na group-containing polyurethane resin: 0.6 parts

-   -   (weight-average molecular weight: 70,000, SO₃Na group: 0.1        meq/g)

2,3-dihydroxynaphthalene: 0.6 parts

Cyclohexanone: 23.0 parts

Protrusion forming agent liquid

Colloidal silica (average particle size: 120 nm): 2.0 parts

Methyl ethyl ketone: 8.0 parts

Other components

Stearic acid: 3.0 parts

Stearic acid amide: 0.3 parts

Butyl stearate: 6.0 parts

Methyl ethyl ketone: 110.0 parts

Cyclohexanone: 110.0 parts

Polyisocyanate (CORONATE (registered trademark) L manufactured by TosohCorporation): 3.0 parts

List of Non-Magnetic Layer Forming Composition

Non-magnetic inorganic powder

α-iron oxide (average particle size: 10 nm, BET specific surface area:75 m²/g): 100.0 parts

Carbon black (average particle size: 20 nm) 25.0 parts

SO₃Na group-containing polyurethane resin (weight-average molecularweight: 70,000, SO₃Na group content: 0.2 meq/g): 18.0 parts

Stearic acid: 1.0 part

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

List of Back Coating Layer Forming Composition

Fine particle carbon black (average particle size: see Table 1): 100.0parts

Coarse particle carbon black (average particle size: see Table 1): seeTable 1

α-iron oxide (average particle size: see Table 1): see Table 1

α-alumina (average particle size: 0.20 μm): 5.0 parts

Nitrocellulose resin: 55.0 parts

Polyurethane resin 40.0 parts

Copper oleate: 0.1 parts

Copper phthalocyanine: 0.2 parts

Preparation of Magnetic Layer Forming Composition

A magnetic layer forming composition was prepared by the followingmethod.

Various components of the magnetic liquid were dispersed (beaddispersion) for 24 hours using a batch type vertical sand mill toprepare a magnetic liquid. As dispersed beads, zirconia beads having abead diameter of 0.5 mm were used.

Various components of the above abrasive liquid were mixed and then themixture was put in a horizontal beads mill dispersing device togetherwith zirconia beads having a bead diameter of 0.3 mm, and the beadvolume/(abrasive liquid volume+bead volume) was adjusted to be 80%, anda beads mill dispersion process was performed for 120 minutes. Theliquid after the process was taken out and subjected to ultrasonicdispersion filtration process using a flow type ultrasonic dispersionfiltration device. Thereby, an abrasive liquid was prepared.

The prepared magnetic liquid and abrasive liquid, and the protrusionforming agent liquid and other components were put into a dissolverstirrer and stirred for 30 minutes at a circumferential speed of 10m/sec, and subjected to processes of 3 passes at a flow rate of 7.5kg/min by a flow type ultrasonic dispersing device, and then a magneticlayer forming composition was prepared by filtration through a filterhaving a pore diameter of 1 μm.

Preparation of Non-Magnetic Layer Forming Composition

Various components of the non-magnetic layer forming composition weredispersed using zirconia beads having a bead diameter of 0.1 mm by abatch type vertical sand mill for 24 hours, and then filtered using afilter having an average pore diameter of 0.5 μm. Thereby, anon-magnetic layer forming composition was prepared.

Preparation of Back Coating Layer Forming Composition

After dispersing various components of the back coating layer formingcomposition using a sand mill for 120 minutes, 15.0 parts ofpolyisocyanate was added, and the mixture was filtered using a filterhaving a pore diameter of 1 μm. Thereby, a back coating layer formingcomposition was prepared.

Manufacturing of Magnetic Tape

The non-magnetic layer forming composition prepared in the above sectionwas applied onto a surface of a biaxially stretched polyethylenenaphthalate support having a thickness of 5.0 μm and was dried so that athickness after drying becomes 400 nm, and thus a non-magnetic layer wasformed. After that, the magnetic layer forming composition prepared inthe above section was applied onto a surface of the non-magnetic layerso that a thickness after drying becomes 70 nm, and thus a magneticlayer forming composition was formed. While this coating layer of themagnetic layer forming composition is in a wet (undried) state, avertical orientation treatment was performed in which a magnetic fieldof a magnetic field intensity of 0.3 T was applied in a directionperpendicular to a surface of the coating layer, and then the surface ofthe coating layer was dried. Thereafter, the back coating layer formingcomposition prepared in the above section was applied onto an oppositesurface of the support so that the thickness after the drying becomes0.4 μm, and then was dried. Thereby, a magnetic tape original roll wasmanufactured.

The manufactured magnetic tape original roll was subjected to acalendering treatment (surface smoothing treatment) by a calender formedof only metal rolls at a speed of 100 m/min, a linear pressure of 294kN/m (1 kg/cm is 0.98 kN/m), and a surface temperature of a calenderingroll of 100° C. Then, a heat treatment was performed in an environmentof an atmosphere temperature shown in Table 1 for the time shown inTable 1. After the heat treatment, the magnetic tape original roll wasslit by a cutter to obtain a magnetic tape having ½ inches width. Whilerunning this magnetic tape between a feeding roller and a winding roller(running speed of 120 m/min, tension: see Table 1), a blade polishingand a dry wiping treatment of the magnetic layer surface were performedin this order. Specifically, a sapphire blade and a dry wiping material(Toraysee manufactured by TORAY INDUSTRIES, INC. (registered trademark))were disposed between the two rollers, and the sapphire blade waspressed against the magnetic layer surface of the magnetic tape runningbetween the two rollers to perform the blade polishing, and then the drywiping treatment on the magnetic layer surface was performed by the drywiping material. Thereby, the blade polishing and the dry wipingtreatment were each performed once on the magnetic layer surface.

Thus, a magnetic tape of Example 1 was obtained.

Examples 2 to 12, Comparative Examples 1 to 6, and Reference Examples 1and 2

A magnetic tape was manufactured in the same manner as in Example 1except that various conditions were changed as shown in Table 1.

In Examples 9 to 12, the magnetic tape original roll manufactured in thesame manner as in Example 1 except that various conditions were changedas shown in Table 1 was subjected to a calendering treatment (surfacesmoothing treatment) by a calender formed of only metal rolls at a speedof 100 m/min, a linear pressure of 294 kN/m, and a surface temperatureof a calendering roll of 100° C. Then, a heat treatment was performed inan environment of an atmosphere temperature shown in Table 1 for thetime shown in Table 1. After the heat treatment, the magnetic tapeoriginal roll was slit by a cutter to obtain a magnetic tape having ½inches width. While running this magnetic tape between a feeding rollerand a winding roller (running speed of 120 m/min, tension: see Table 1),a blade polishing, a dry wiping treatment, and a methyl ethyl ketonewiping treatment of the magnetic layer surface were performed in thisorder. Specifically, a sapphire blade, a dry wiping material (Torayseemanufactured by TORAY INDUSTRIES, INC. (registered trademark)), and awiping material (Toraysee manufactured by TORAY INDUSTRIES, INC.(registered trademark)) infiltrated with methyl ethyl ketone weredisposed between the two rollers, and the sapphire blade was pressedagainst the magnetic layer surface of the magnetic tape running betweenthe two rollers to perform the blade polishing, and then the dry wipingtreatment on the magnetic layer surface was performed by the dry wipingmaterial. After that, the methyl ethyl ketone wiping treatment on themagnetic layer surface was performed by the wiping material infiltratedwith methyl ethyl ketone. Thereby, the blade polishing, the dry wipingtreatment, and the methyl ethyl ketone wiping treatment were eachperformed once on the magnetic layer surface.

Evaluation Method

(1) Number of Protrusions on Back Coating Layer

For each magnetic tape, the number of protrusions having a height of 50nm or more and less than 75 nm and the number of protrusions having aheight of 75 nm or more on the back coating layer surface were obtainedby the following method.

(2) Spacing Difference (S_(after)−S_(before)) Before and After MethylEthyl Ketone Cleaning

Using a tape spacing analyzer (TSA; manufactured by Micro Physics), aspacing difference (S_(after)−S_(before)) before and after methyl ethylketone cleaning was obtained by the following method.

Two sample pieces having a length of 5 cm were cut from the magnetictape, and one sample piece was not subjected to methyl ethyl ketonecleaning, and this a spacing (S_(before)) was obtained by the followingmethod. The other sample piece was subjected to methyl ethyl ketonecleaning by the method described above, and then a spacing (S_(after))was obtained by the following method.

In a state where a glass plate (a glass plate manufactured by Thorlabs,Inc. (model number: WG10530)) provided in TSA is disposed on themagnetic layer surface of the magnetic tape (specifically, the samplepiece), using a urethane hemisphere provided in the TSA as a pressingmember, the hemisphere was pressed against the back coating layersurface of the magnetic tape at a pressure of 0.5 atm. In this state,white light was emitted from a stroboscope provided in the TSA to acertain area (150,000 to 200,000 μm²) on the magnetic layer surface ofthe magnetic tape through a glass plate, and the obtained reflectedlight was received by a charge-coupled device (CCD) through aninterference filter (a filter that selectively transmits light having awavelength of 633 nm), and thus an interference fringe image generatedby an unevenness of this area was obtained.

This image was divided into 300,000 points to obtain a distance(spacing) from the magnetic tape side surface of the glass plate to themagnetic layer surface of the magnetic tape of each point, and this wasused as a histogram. Thus, a difference (S_(after)−S_(before)) wasobtained by subtracting a mode value S_(before) of the histogramobtained for the sample piece without methyl ethyl ketone cleaning froma mode value S_(after) of the histogram obtained for the sample pieceafter methyl ethyl ketone cleaning.

(3) Spacing Difference (S_(reference)−S_(before)) before and aftern-Hexane Cleaning (reference value)

One sample piece having a length of 5 cm was further cut from themagnetic tape and was cleaned in the same manner as the above exceptthat n-hexane was used instead of methyl ethyl ketone, and then aspacing after n-hexane cleaning was obtained in the same manner asdescribed above. As a reference value, the difference(S_(reference)−S_(before)) between a spacing S_(reference) obtained hereand a spacing S_(before) obtained for the uncleaned sample pieceobtained in the above (2) was obtained.

(4) Dropout

Dropout was measured using a ½ inches reel tester with a fixed head. Asignal having a linear recording density of 300 kfci was recorded usinga recording head, and was reproduced by a giant magnetoresistive (GMR)head having a track width of 1 μm. The unit kfci is a unit of a linearrecording density (cannot be converted into SI unit system). The numberof signal dropouts of a length of 0.4 μm or more in length at an outputdrop of 40% or more of an average output was detected, and the numberper 1 m of tape length (per measurement area 1 mm² (=track width (1μm)×tape length (1 m))) was adopted as dropout.

The above results are shown in Table 1 (Table 1-1 to Table 1-3).

TABLE 1-1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Magnetic Ferromagnetic powder SrFe1 SrFe1 SrFe1 SrFe2 SrFe2 ε-Iron oxidelayer Hk (kOe) 25 25 25 19 19 30 Back Fine particle Average particlesize 13 13 13 13 13 13 coating carbon black (nm) layer Coarse Averageparticle size 101 270 101 270 particle (nm) carbon black Content (part)4.0 4.0 4.0 4.0 α-Iron oxide Average particle size 10 10 (nm) Content(part) 4.0 4.0 Number of Height of 50 nm or more 5 310 630 310 630 5protrusions and less than 75 nm (piece/6400 μm²) Height of 75 nm or more0 100 180 100 180 0 Heat Temperature 60° C. 60° C. 60° C. 60° C. 60° C.60° C. treatment Time 24 hours 24 hours 24 hours 24 hours 24 hours 24hours Tension (N) 0.294 0.294 0.294 0.294 0.294 0.294 Blade polishingand dry wiping treatment Once Once Once Once Once Once Methyl ethylketone wiping treatment Not Not Not Not Not Not performed performedperformed performed performed performed (Reference value) 2.0 2.0 2.02.0 2.0 2.0 Spacing difference (S_(reference) − S_(before)) before andafter n-hexane cleaning (nm) Spacing difference (S_(after) − S_(before))before and 20.0 20.0 20.0 20.0 20.0 20.0 after methyl ethyl ketonecleaning (nm) Dropout (piece/mm²) 4 220 450 150 350 8

TABLE 1-2 Example 7 Example 8 Example 9 Example 10 Example 11 Example 12Magnetic Ferromagnetic powder ε-Iron oxide ε-Iron oxide SrFe1 ε-Ironoxide SrFe1 ε-Iron oxide layer Hk (kOe) 30 30 25 30 25 30 Back Fineparticle Average particle size 13 13 13 13 13 13 coating carbon black(nm) layer Coarse Average particle size 101 270 270 270 particle (nm)carbon black Content (part) 4.0 4.0 4.0 4.0 α-Iron oxide Averageparticle size 10 10 (nm) Content (part) 4.0 4.0 Number of Height of 50nm or more 310 630 630 630 5 5 protrusions and less than 75 nm(piece/6400 μm²) Height of 75 nm or more 100 180 180 180 0 0 HeatTemperature 60° C. 60° C. 70° C. 70° C. 70° C. 70° C. treatment Time 24hours 24 hours 36 hours 36 hours 36 hours 36 hours Tension (N) 0.2940.294 0.294 0.294 0.294 0.294 Blade polishing and dry wiping treatmentOnce Once Once Once Once Once Methyl ethyl ketone wiping treatment Notperformed Not performed Once Once Once Once (Reference value) 2.0 2.02.0 2.0 2.0 2.0 Spacing difference (S_(reference) − S_(before)) beforeand after n-hexane cleaning (nm) Spacing difference (S_(after) −S_(before)) before and after 20.0 20.0 12.0 12.0 12.0 12.0 methyl ethylketone cleaning (nm) Dropout (piece/mm²) 300 580 120 130 2 4

TABLE 1-3 Refer- Refer- Compar- Compar- Compar- Compar- Compar- Compar-ence ence ative ative ative ative ative ative Exam- Exam- Exam- Exam-Exam- Exam- Exam- Exam- ple 1 ple 2 ple 1 ple 2 ple 3 ple 4 ple 5 ple 6Magnetic Ferromagnetic powder BaFe BaFe SrFe1 SrFe1 SrFe2 SrFe2 ε-Ironoxide ε-Iron oxide layer Hk (kOe) 13 13 25 25 19 19 30 30 Back Fineparticle Average particle 13 43 25 43 25 43 25 43 coating carbon blacksize (nm) layer Coarse Average particle 101 101 270 101 270 101 270 101particle size (nm) carbon black Content (part) 4.0 4.0 4.0 4.0 4.0 4.04.0 4.0 Number of Height of 50 nm 310 1350 920 1350 920 1350 920 1350protrusions or more and less (piece/6400 than 75 nm μm²) Height of 75 nm100 465 260 465 260 465 260 465 or more Heat Temperature 60° C. 60° C.60° C. 60° C. 60° C. 60° C. 60° C. 60° C. treatment Time 24 hours 24hours 24 hours 24 hours 24 hours 24 hours 24 hours 24 hours Tension (N)0.294 0.294 0.294 0.294 0.294 0.294 0.294 0.294 Blade polishing and drywiping treatment Once Once Once Once Once Once Once Once Methyl ethylketone wiping treatment Not Not Not Not Not Not Not Not performedperformed performed performed performed performed performed performed(Reference value) 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Spacing difference(S_(reference) − S_(before)) before and after n-hexane cleaning (nm)Spacing difference (S_(after) − S_(before)) before and 20.0 20.0 20.020.0 20.0 20.0 20.0 20.0 after methyl ethyl ketone cleaning (nm) Dropout(piece/mm²) 110 590 760 1050 620 800 890 1290

The magnetic tapes of Examples 1 to 12 and Comparative Examples 1 to 6are magnetic tapes that includes a magnetic layer includingferromagnetic powder selected from the group consisting of hexagonalstrontium ferrite powder and ε-iron oxide powder and a back coatinglayer. As shown in Table 1, many dropouts occurred in the magnetic tapesof Comparative Examples 1 to 6, whereas in the magnetic tapes ofExamples 1 to 12 in which the number of protrusions on the back coatinglayer surface having a height of 50 nm or more and less than 75 nm iswithin the range described above, dropout could be reduced.

On the other hand, in a magnetic tape of Reference Example 2 which is amagnetic tape including hexagonal barium ferrite powder in the magneticlayer, even though the number of protrusions having a height of 50 nm ormore and less than 75 nm on the back coating layer surface greatlyexceeds the range described above, occurrence of dropout was smallerthan that of the magnetic tapes of Comparative Examples 1 to 6. This isconsidered to indicate that in a magnetic tape that includes a magneticlayer including ferromagnetic powder selected from the group consistingof hexagonal strontium ferrite powder and ε-iron oxide powder,protrusions having a height of 50 nm or more and less than 75 nm on theback coating layer surface greatly affect dropout.

As shown in Table 1, there is no correlation between the value of thespacing difference (S_(reference)−S_(before)) before and after n-hexanecleaning and the value of the spacing difference (S_(after)−S_(before))before and after methyl ethyl ketone cleaning.

One aspect of the present invention is effective in a technical field ofa magnetic tape for high-density recording.

What is claimed is:
 1. A magnetic tape comprising: a non-magneticsupport; a magnetic layer that includes ferromagnetic powder on onesurface side of the non-magnetic support; and a back coating layer thatincludes non-magnetic powder on the other surface side of thenon-magnetic support, wherein the ferromagnetic powder is ferromagneticpowder selected from the group consisting of hexagonal strontium ferritepowder and ε-iron oxide powder, the number of protrusions having aheight of 50 nm or more and less than 75 nm on a surface of the backcoating layer is 700 pieces/6400 μm² or less, and a differenceS_(after)−S_(before) between a spacing S_(after) measured on a surfaceof the magnetic layer by optical interferometry after methyl ethylketone cleaning and a spacing S_(before) measured on the surface of themagnetic layer by optical interferometry before methyl ethyl ketonecleaning is more than 0 nm and 15.0 nm or less.
 2. The magnetic tapeaccording to claim 1, wherein the number of protrusions having a heightof 75 nm or more on the surface of the back coating layer is 200pieces/6400 μm² or less.
 3. The magnetic tape according to claim 1,further comprising: a non-magnetic layer including non-magnetic powderbetween the non-magnetic support and the magnetic layer.
 4. A magnetictape cartridge comprising: the magnetic tape according to claim
 1. 5.The magnetic tape cartridge according to claim 4, wherein the number ofprotrusions having a height of 75 nm or more on the surface of the backcoating layer is 200 pieces/6400 μm² or less.
 6. The magnetic tapecartridge according to claim 4, wherein the magnetic tape furthercomprises a non-magnetic layer including non- magnetic powder betweenthe non-magnetic support and the magnetic layer.
 7. A magnetic recordingand reproducing apparatus comprising: the magnetic tape according toclaim 1; and a magnetic head.
 8. The magnetic recording and reproducingapparatus according to claim 7, wherein the number of protrusions havinga height of 75 nm or more on the surface of the back coating layer is200 pieces/6400 μm² or less.
 9. The magnetic recording and reproducingapparatus according to claim 7, wherein the magnetic tape furthercomprises a non-magnetic layer including non-magnetic powder between thenon-magnetic support and the magnetic layer.